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STUDIES ON THE KINETICS AND MECHANISM OF THE OXIDATION OF CARBOHYDRATES IN THE ABSENCE AND PRESENCE OF SURFACTANTS
ABSTRACT T H E S I S
S U B M I T T E D FOR T H E A W A R D O F T H E OEC3REE O F
Soctor of pi|tl0fii0pl)B IN
CHEMISTRY
BY
MOHD. SAJID ALI
DEPARTMENT OF CHEMISTRY ALIGARH MUSLIM UNIVERSITY
ALIGARH (INDIA)
2006
A<BST<KACT
Carbohydrates are widely distributed in plants and animals, where they
fulfill both structural and metabolic roles. They are of three types, namely,
mono-, oligo-, and polysaccharides. Monosaccharides are fundamental
biomolecules in that they are building blocks of polysaccharides. They are
building blocks of nucleotides also and, hence, of nucleic acids and of the
chemical ADP/ATP energy storage system.
Oxidation and reduction reactions of sugars play key roles in
biochemistry. Oxidation of sugars provides energy for organisms to carry out
their life processes. The physiological and microbiological activities of
carbohydrates depend largely in their redox behavior. Oxidation of
monosaccharides by different oxidizing agents are, therefore, of special
importance due to their biological relevance.*"^ Due to multihydroxy
functionality of saccharides they can chelate and coordinate to many metal ions.
Cerium(IV) is used as an oxidant not only in analysis, but also in synthetic
organic chemistry. Kinetic and mechanistic aspects of cerium(IV) oxidation of
monosaccharides has also been studied ' " '' but no attempt has been made on
such redox systems in presence of micelles.
The chemical approach to biological problems through investigations of
models rests upon the ability of the chosen system to mimic some functions of
the biological ensemble. Surfactants in aqueous media have been extensively
used as model systems. Surfactants are amphipathic molecules which have
distinct hydrophobic and hydrophilic regions. Over a narrow concentration
range, defined as the critical micelle concentration, or cmc, surfactants
dynamically associate to form large molecular aggregates, called micelles.
Rates of numerous organic and inorganic reactions are affected by
micelles in aqueous solutions.'' Catalysis or inhibition is the consequence of
substrate solubilization in the micellar pseudophase. Rate effects can be
attributed to electrostatic, hydrophobic, electrophilic and/or nucleophilic
interactions with the resultant alteration of free energy of activation for the
overall process. Interest in micellar chemistry has been prompted by the
proposed similarities between the structures of the globular proteins and
spherical micelles and between micellar and enzymatic catalyses.
Therefore, the present thesis entitled 'Studies on the Kinetics and
Mechanism of the Oxidation of Carbohydrates in the Absence and Presence
of Surfactants' is exclusively devoted to study the role of cationic and anionic
micelles on reaction of cerium(IV) and carbohydrates. The surfactants used in
this study are CTAB (cationic) and SDS (anionic) and the carbohydrates are two
aldopentoses (D(+)xylose and L(+)arabinose), two aldohexoses (D(+)glucose
and D(+)mannose), and two ketohexoses (D(-)fructose and L(-)sorbose). There
are three chapters in the thesis, namely (i) Chapter 1 - Introduction; (ii) Chapter
2 - Experimental; and (iii) Chapter 3 - Results and Discussion.
Chapter 1 includes introduction about carbohydrates, their importance,
properties, chelating ability and reactivity towards metal ions, especially with
cerium(IV). A brief account of the kinetics and mechanism of the cerium(IV)—
carbohydrate reactions studied by different workers is also given.
The classification of surfactants, their uses, behavior and the pseudophase
model and its applicability to the micellar catalyzed reactions is also provided.
The chapter ends with the statement of the problem which suggests the
importance of this study.
All the experimental details are described in Chapter 2. The materials
used, their structure and formulas, sources and purities are given along with the
method for the preparation of solutions and kinetic measurements. The method
of cmc determination and procedure for the characterization and identification of
products and stoichiometric determinations are also detailed in this chapter.
Cerium(IV) is reduced to cerium(III), which is colorless at the maximum
wavelength of cerium(IV), i.e., 385nm. Therefore, examples of the spectra of the
reaction product are also given in Chapter 2.
Chapter 3 - Results and Discussion, as the name implies, covers all the
results obtained with their discussion. The oxidative degradation of
carbohydrates by cerium(IV) has been found to be slow in aqueous H2SO4
medium with the evidence of autocatalysis. The effect of varying the
concentrations of cerium(IV), carbohydrate, sulfuric acid, SO4 , HSO4 and
surfactant was seen to elaborate their role in the cerium(IV)—carbohydrate
redox reactions. The values of pseudo-first-order rate constants were
independent on the initial concentrations of oxidant (cerium(IV)) indicating the
first-order dependence of the reaction rate on [Ce(IV)]. The plots of rate
constants versus [reductant] were linear with zero intercepts, clearly suggesting
the first-order dependence of rate constants on carbohydrates. The pseudo-first-
order rate constants decreased with increase in concentrations of H2SO4 and
HS04~ while a rate increase was found with increase in [S04^~]. Effect of
temperature on the reaction rate was studied to obtain the values of
thermodynamic parameters. As regards the effect of surfactants, the rate
constants increased as the concentration of cationic CTAB increased whereas
anionic SDS has no effect (which may be due to the electrostatic repulsion
between the negative head group of SDS and the reactive species of cerium(IV)).
Lower values of activation energies for the reactions in CTAB medium as
compared to aqueous medium confirm the catalysis.
Existence of various forms of monosaccharides and cerium(IV) has been
discussed and p-anomer of pyranoid form is envisaged to be involved in the
oxidation of monosaccharides. The rate increasing effect of CTAB and
constancy of rates on varying [SDS] showed the participation of negatively
charged species of cerium(IV). On the basis of [H2SO4] and [HSO4"]-
dependencies, species Ce(S04)3^~ and/or HCe(S04)3~ are proposed as the
reactive species.
A probable mechanism is presented and discussed. The reactions start
with formation of a complex between cerium(IV) and carbohydrate which
undergoes decomposition in rate determining step. The oxidation products of the
reactions are Ce(III), lactones, and aldonic acids (aldoses)/formaldehyde
(ketoses).
The Mechanism for
(A) Aldoses
R
P-anomer "
_0H + Ce(IV) =
R
Kd. |3-anomer Ce(IV)
CI
CI - ^ \^^^^^^^^0 + Ce(III) + H-
Radical H
R fast
Radical +Ce(IV) — \ V
0H/H2P
' H+
^\^>^\ + Ce(III) + H
Lactone 0
^ - ^ ^ O H
Aldonic acid anion
(1)
(2)
(3)
(4)
(R = — H (for aldopentoses) or -CH2OH (for aldohexoses) and Ce(IV) denotes
the kinetically active Ce(IV)-species)
(B)Ketoses
\ i : : ^ O H + Ce(IV) ^
„ CH20H p-anomer ^
K^ P-anomer Ce(IV)
CI
(5)
CI -^^-* X^-^- '- 'Oi ^ ^^("^^ ^ CH2OH + H+ (6)
Lactone ^ Radical
Radical + Ce(IV) ^ ^ Ce(III) + HCHO + H' (7)
Effect of the variables [oxidant], [reductant], [H2SO4], and temperature
were also studied in presence of CTAB and it was found that the dependence of
rate constants on all variables was same as in case of aqueous medium. Thus, the
same mechanism operative in the aqueous medium is being followed in the
CTAB micellar medium too.
The CTAB micelle-catalyzed kinetic results are interpreted by the
Menger-Portnoy'^ model where cerium(IV) in water associates with micellized
surfactant (Dn) giving micellized cerium(IV) and the reaction occurring in the
aqueous and micellar pseudophases with first-order rate constants (k'w and k'm).
The first-order rate constant for the overall reaction (kvp) is given by the
following equation
k'w+k'„,Ks[Dn] kvp - (8)
1 + Ks [D„]
which, on rearrangement gives
1 1 1 = + (9)
(k'w - k^) (k'w - k-J (k'w - k 'J Ks [D„]
The values of k'm and Kg were evaluated from the slopes and intercepts of the
plots of l/(k'w- k^) versus l/[Dn].
The positive catalytic effect of CTAB micelles on the Ce(IV)-
carbohydrate redox reactions has been explained in the following manner. The
chemically active anionic Ce(IV)-species gets associated with the cationic
micelles. The second reactant, carbohydrate, has no hydrophobicity due to the
presence of hydrophilic —OH groups. As the reaction proceeds through the
formation of a complex (see Eq. (l)/(5)), the associated Ce(IV>-species may
form complex CI at the Stem and Gouy-Chapman layers' junctural region. The
complex may now orient in a manner suitable for continuing the reaction. A
possible arrangement (although highly schematic) could be as shown in Fig. 1.
Fig. 1: Schematic model showing probable location of reactants for the ionic
micellar catalyzed redox reaction between cerium(IV) and
carbohydrates.
Added inorganic salts (Na2S04, NaNOs, NaCl) inhibit the CTAB-
catalyzed reaction that may be due to the exclusion of reactive species of
cerium(IV) from the reaction site. The inhibitory power increases in the order
Cr<N03"<S042".
On the basis of the second -order rate constant values for the reactivity of
carbohydrates with cerium(IV), it is inferred that presence of—OH, —CHO and
ketonic groups increase the reducing power in the order aldohexoses <
aldopentoses < ketohexoses. The trend shows that the oxidation by cerium(IV)
seemingly depends on the number of —OH groups, stereochemistry and the
chelating ability of the monosaccharides. D(-)fructose has greater tendency to
reduce cerium(IV) in comparison to L-sorbose and other monosaccharides
(L(-)sorbose > L(+)arabinose > D(+)xylose > D(+)mannose > D(+)glucose). It
is interesting to note that the oxidation rates of various monosaccharides studied
are of the same order. This means that these sugars are oxidized by a common
mechanism, i.e., cerium forming a complex with C-1 hydroxyl group of the
sugar prior to its rate-limiting disproportionation to a free radical.
10
References
1. L. F, Sala, A. F. Cirelli, R. M. de Lederkremer, J. Chem. Soc, Perkin
Trans. 2, 1977, 685.
2. J. Barek, A. Berka, A. Pokorna-Hladikova, Collect. Czech. Chem.
Commun., 1982,47,2466.
3. M. Gupta, S. K. Saha, P. Banerjee, J. Chem. Soc, Perkin Trans. 2, 1988,
1781.
4. S. Signorella, L. Ciullo, R. Lafarga, L. F. Sala, New J. Chem., 1996, 20,
989.
5. C. R. Pottenger, D. C. Johnson, J. Polym. Sci. Part A-1, 1970, 8, 301.
6. R. N. Mehrotra, E. S. Emis, J. Org. Chem., 1974, 39, 1788.
7. A. G. Fadnis, Carbohydr. Res., 1986, 146, 97, and the references cited
therein.
8. P. 0 .1 . Virtanen, R. Lindroos, E. Oikarinen, J. Vaskuri, Carbohydr. Res.,
1987, 167, 29, and the references cited therein.
9. K. K. Sen Gupta, S. Sen Gupta, A. Mahapatra, J. Carbohydr. Chem.,
1989, 8, 713, and the references cited therein.
10. A. Roy, A. K. Das, Indian J. Chem., 2002, 41A, 2468, and the references
cited therein.
11. (a) J. H. Fendler, E. J. Fendler, ''Catalysis in Micellar and
Macromolecular Systems", Academic Press, New York, 1975; (b) J. H.
Fendler, ""Membrane Mimetic Chemistry", Wiley-Interscience,
New York, 1982.
12. F. M. Menger, C. E. Portnoy, J. Am. Chem. Soc, 1967, 89, 4698.
STUDIES ON THE KINETICS AND MECHANISM OF THE OXIDATION OF CARBOHYDRATES IN THE ABSENCE AND PRESENCE OF SURFACTANTS
THESIS
SUBMITTED FOR THE AWARD OF THE DEGREE OF
Snctnr nf tpiitlnfinpim IN
CHEMISTRY
BY
MOHD. SAJID ALI
DEPARTMENT OF CHEMISTRY ALIGARH MUSLIM UNIVERSITY
ALIGARH (INDIA)
2006
I l l
To My Parents
PROF. KABIR-UD-DIN CHAIRMAN DEPARTMENT OF CHEMISTRY ALIGARH MUSLIM UNIVERSITY ALIGARH-202002 (U. P.) INDIA. E-mail: [email protected]
EXT. (0571) -2703515 Int. 3353,3351
Certificate
Dated:. \.^\f>.k\..0. C
This is to certify that the thesis entitled "Studies on the Kinetics
and Mechanism of the Oxidation of Carbohydrates in the Absence and
Presence of Surfactants" is the original work carried out by Mr. Mohd.
Sajid Ali under my supervision and is suitable for submission for the
award of Ph.D. degree in Chemistry.
VC (Prof. Kabir-ud-Din)
Acknowledgements
"No effort takes finat sfiape unCess HeCp is rendered from various comers. I too
received HeCp and guidance from many quarters. (But initiaCCy I am most indebted to the grace
of "One VniversaC (Being" who inspires entire humanity towards ^owCedge, truth and
etemaCjoys. I Bring aCC the praises and commendation to the (^mighty for providing me with
strength during the wor^ofmy research, which heCpedme to overcome the trouBCes on the
toiCsome journey.
I v/ish to express my sincere and deep sense of gratitude to my esteemed supervisor
(Prof %fl.Bir-ud-(Din, Chairman, (Department of Chemistry, JlCigath !Mus[im Vniversity,
^Cigarh for his e:)qpert guidance, persistent interest, schotarCy criticism and constant
encouragement throughout the course of this study.
I am deepCy indeBtedto (Dr. Zaheer Xfian, (Reader, (Department of Chemistry, Jamia
CMiCCia IsCamia, ^Kew (DeChi,for his ndness and support.
I expend my sincere than^ to (Dr. Sanjeev Xumar, (Dr. 9d. Z. Ji. (Rflfiquee, <Dr. M.
A^am, and (Dr. JlndkeB Z. !Naqvi.
My deepest thanH^ goes to (Dr. Ziya Ahmad 'Kfian for his support and
encouragement.
9A.y thanks and Best wishes are due to Mr. S. M. Sha^eef IquBaC, (Dr. (Da^sha
Sharma, 9/trs. M/aseefa Tatma, Mrs. Vmme SaCma, (Deepti Sharma, Jiuzhat ^uCC, Sayem
A(am, JVeeCam J{azoor,Mohd. ACtaf, O^aved Azam, Suraiya, and Mohd. (DaBi AR A^^
Ahmadi.
I am thankfuCto a[[ of my friends especially, Tanweer Ahmad, Adif Tfussain, and
Hafiz Iqtidar Ahmad for their day-to-day heCp andvaCuaBCe suggestions.
Words cannot express my deepest thanks and gratitude to my nana, my father, my
mother, khaCa jaan. Brothers, and sisters-in-law who stood with me in adversity and
prosperity with continuous strive and endeavor and whose Cove, patience and sacrifice are
vested in every day of my fife.
Mr. SaCim iJddin and Mr. Z. Ahmad deserve praise for their work^and cooperation.
MOHD. SAJID ALI
List of Publications
1. Effect of anionic and cationic micelles on the oxidation of D-glucose by
cerium(IV) in presence of H2SO4
Kabir-ud-Din, Mohd. Sajid Ali, Zaheer Khan
Colloid Polym. Sci., 2005, 284, 10-18.
2. Oxidation of L(+)arabinose by cerium(IV) in presence of anionic and cationic
micelles.
Kabir-ud-Din, Mohd. Sajid Ali, Zaheer Khan
Indian J. Chem., 2005,44A, 2462-2469.
3. Effect of surfactant micelles on the kinetics of oxidation of D-fiructose by
cerium(IV) in sulfuric acid medium.
Kabir-ud-Din, Mohd. Sajid Ali, Zaheer Khan
Int. J. Chem. Kinet., 2006,38, 18-25.
4. Kinetics of the oxidative degradation of D-xylose in presence and absence of
cationic and anionic surfactants.
Kabir-ud-Din, Mohd. Sajid Ali, Zaheer Khan
Colloid Polym. Sci., 2006, 284, 627-633.
5. Micelle-assisted cerium(IV) oxidation of L-sorbose in aqueous sulfuric acid.
Kabir-ud-Din, Mohd. Sajid Ali, Zaheer Khan
Int. J. Chem. Kinet., (Revised manuscript submitted).
6. Micelle catalyzed oxidation of D-mannose by cerium(IV) in sulfuric acid.
Kabir-ud-Din, Mohd. Sajid Ali, Zaheer Khan
Inorg. React. Mech., (Communicated).
CONTENTS Page
CHAPTER 1
CHAPTER 2
INTRODUCTION
A. Oxidation of carbohydrates
B. Cerium(IV) oxidation of carbohydrates
C. Surfactant and surfactant micelles
D. The pseudophase model
E. Statement of the problem
References
EXPERIMENTAL
A. Materials
B. Preparation of solutions
C. Kinetic measurements
D. Determination of cmc
E. Product analysis
References
CHAPTER 3 RESULTS AND DISCUSSION
A. RESULTS
Effect of [oxidant] on the rate
Effect of [reductant] on the rate
Effect of [H2SO4] on the rate
Effect of [S0/~] on the rate
1-37
2
6
10
20
25
27
38-52
39
39
39
43
46
52
53-150
54
Effect of [HSO4I on the rate
Effect of temperature on the rate
Effect of [surfactant] on the rate
Effect of [oxidant], [reductant], [H2SO4] and temperature
on the rate in micellar medium
Effect of [salt] on the rate in micellar medium
B. DISCVSSIOI^ 126
(i) in the absence of CTAB 126
(ii) in the presence of CTAB 133
References 148
im:<Rp(DVcTio^
A. Oxidation of Carbohydrates
The simple organic compounds from which living organisms are
constructed are unique to life and do not otherwise occur on the earth today,
except as products of biological activity. These building-block compounds,
called biomolecules, were selected during the course of biological evolution for
their fitness in performing specific cell functions. They are identical in all
organisms. Biomolecules are related to each other and interact in a kind of
molecular "game" or logic. The size, shape and chemical reactivity of
biomolecules enable them not only to serve as building blocks of the intricate
structure of cells, but also to participate in their dynamic, self-sustaining
transformation of energy and matter. Biomolecules must therefore be examined
from two viewpoints, that of the chemist and that of the biologist.
Living things are composed of lifeless molecules. When these molecules
are isolated and examined individually, they conform to all the physical and
chemical laws that describe the behavior of inanimate matter.
Carbohydrates are one of the four major classes of biomolecules along
with proteins, nucleic acids, and lipids. Carbohydrates are aldehyde or ketone
compounds with multiple hydroxyl groups which may be classified as
monosaccharides, oligosaccharides, and polysaccharides; the term saccharide is
derived from the Greek word for sugar. Monosaccharides are single
polyhydroxyaldehyde (known as aldoses, e.g., glucose) or polyhydroxyketone
units (known as ketoses, e.g., fructose), whereas oligosaccharides consist of two
to ten monosaccharide units joined together by glycosidic linkages. Sucrose and
lactose are disaccharides, since they are each made up of two monosaccharide
units. Polysaccharides, as the name implies, may contain hundreds of
monosaccharide units.
Carbohydrates make up most of the organic matter on earth because of
their extensive roles in all forms of life. Carbohydrates serve as energy stores,
fuels, and metabolic intermediates. Ribose and deoxyribose sugars form part of
the structural framework of RNA and DNA. Carbohydrates are linked to many
proteins (glycoproteins) and lipids (glycolipids), where they play key role in
mediating interactions among cells and interaction between cells and other
elements in cellular environment.
Carbohydrates provide the skeletal framework for tissues and organs of
the human body and serve as lubricants and support elements of connective
tissue. Major energy requirements of the body are met by dietary carbohydrates.
They confer biological specificity and provide recognition elements on cell
membranes.
Carbohydrates form the most abundant group of the natural products and
1 _ Q
are found in all classes of living organisms. They serve as a direct link
between the energy of the sun and metabolic energy that is required to sustain
life. In organisms, capable of photosynthesis, solar energy is harvested to derive
reaction in which glucose is synthesized from carbon dioxide and water. The
energy stored in "carbon fixation" process then gradually moves upwards into
the food chain. The living organisms that partake the products of photosynthesis
obtain useful energy by oxidizing the carbohydrates back into carbon dioxide
and water through the process of glycolysis and respiration.
In addition to their pivotal role in metabolism, carbohydrates also play an
important role in many organisms. Some examples of the latter type include
cellulose, chitin, lipopolysaccharide, and the bacterial murein, all of which are
derived from repeating sugar units which may have additional cross-linking
components for rigidity. Furthermore, many biotic secondary metabolites such
as cardioglycosides, macrolide antibiotics, and aminoglycoside antibiotics rely
on the sugar components for solubility and activity. In addition, carbohydrates
are used as convenient precursors for the biosynthesis of other important
building blocks such as aromatic amino acids. Carbohydrates also have many
applications in industrial processes. For example, the food industry uses sucrose
as a sweetening agent, a preservative, and a raw material for fermentation.
Starch is used as a raw material for the manufacture of many goods. Cotton is
still one of the most popular fabrics and an important raw material for the textile
industry. Paper and other derivatives of cellulose are important for the
manufacture of packaging materials and plastics.
Glycosides are compounds formed from a condensation between a
monosaccharide, or monosaccharide residue, and the hydroxyl group of the
second residue that may, or may not be another monosaccharide. Glycosides are
found in many drugs and spices and in the constituents of animal tissues.
A knowledge of the structure and properties of the carbohydrates of
physiologic significance is essential to understanding their fundamental role in
the economy of the mammalian organism. The sugar glucose is the most
important carbohydrate. It is as glucose that the bulk of dietary carbohydrate is
absorbed into the bloodstream or into which it is converted in the liver, and it is
from glucose that all other carbohydrates in the body can be formed.
Carbohydrates exert a wide range of functions in living organisms, and
due to the wide distribution of metals and their complex functions for all forms
of life, metal-carbohydrate interactions are a key for understanding bioinorganic
chemistry, and the study of complexation of carbohydrates to metals is one of
the main objectives of carbohydrate coordination chemistry. Metal complexes of
natural carbohydrates have been attracting interest for many years because these
compounds participate in vitally important processes; they are used for
configurational and conformational analysis, determination, and separation of
sugars. Many carbohydrates easily undergo redox processes. Facile oxidation
can abrogate metal binding, particularly so with high oxidation state transition
metals.
The physiological and microbiological activities of carbohydrates depend
largely in their redox behavior. Oxidation of monosaccharides by different
oxidizing agents are, therefore, of special importance due to their biological
relevance.^''^ Due to multihydroxy functionality of saccharides they can chelate
and coordinate to many metal ions. Besides acting simply as effective
chelators,'^ in many cases they are also reducing agents, e.g., for metal ions such
as Ce(IV), Fe(III)/° Co(III)/WCV), "* depending on the acidity of the medium.
B. Cerium(IV) Oxidation of Carbohydrates
In plant and animal tissues cerium is an important metal element,' it
combines with most of pivotal living active molecules and plays important
physiological function. Cerium(III), the reduction product of cerium(IV), with
suited concentration can activate plant growth and improve the metabolism level
of sugar and grease. For this reason, various fertilizers and fodder containing
cerium(III) are widely applied in China.
Cerium(IV) has been used as an oxidizing agent and an analytical reagent,
especially in an acid medium.' Oxidation of organic compounds with
cerium(IV) are potentially interesting since cerium(IV) is an unusually strong,
one-electron oxidant. Moreover, unique reactions of cerium(IV) with organic
compounds can be expected because of specific coordination properties of the
ion with various organic and inorganic ligands.
The oxidation of organic substances by cerium(IV) reagents is found to
follow different mechanisms, depending on the type of acid media used. The
oxidation potential of the Ce(IV)- Ce(III) couple is markedly ligand dependent,
e.g. the potentials are 1.70 to 1.71, 1.61, 1.44, and 1.28 volts in IN perchloric,
nitric, sulfuric, and hydrochloric acids, respectively/^"^^ The oxidation potential
in hydrochloric acid is probably low (in negative sense), because reaction at the
platinum electrode is not reversible. ^ Increasing the acid concentration from IN
to 8N increases the potential in perchloric acid to 1.87 volts, whereas a decrease
17 1R
to 1.56 and 1.42 volts is observed, respectively, in nitric and sulfuric acids. '
The increase in potential with increasing perchloric acid concentration is in part
attributed to cerium(IV) hydrolysis products. The decrease in potential in
sulfuric acid and nitric acid with increasing acid concentration can be attributed
to complexing of cerium ions with sulfate and nitrate anions. These predictions
have been verified quantitatively for perchloric and sulfuric acid solutions. The
standard potential (E°) in sulfuric acid was calculated to be 1.74 volts^^ when
account was made for bisulfate dissociation and the equilibria (Eqs. (1.1)-(1.3)):
Ce(IV) + HS04" ^ Ce(S04) ^ + H K, = 3500 (1.1) ^
Ce(S04)- + HSO4' ,> Ce(S04)2 + H* Kj = 200 (1.2)"
Ce(S04)2+HS04- ^ Ce(S04)3 " + H K3 = 20 (1.3)"
Scheme 1.1
As regards equilibria in the aqueous H2SO4 media, the studies by
Hardwick and Robertson^^ are very important, but later on Bugaenko and Kuan-
Lin have modified the observation made by Hardwick and Robertson who
proposed the tri-sulfato species as Ce(S04)3^~. From the studies of Bugaenko
and Kuan-Lin, it was established that the tri-sulfato species is HCe(S04)3~.
According to their studies '* in aqueous H2SO4 media (up to ca. 2 mol dm ) the
predominant equilibria are represented by Eqs. (1.1), (1.2), (1.4), and (1.5):
Ce(S04)2+HS04- ^ HCe(S04)3- K5 = 0.6 ±0.1 {\Af'
HCe(S04)3-+H2S04^^=^^ H3Ce(S04)4- K6 = 2±l (l.S) ^
Scheme 1.2
At higher concentrations of H2SO4, the concentration of the H3Ce(S04)4~
species increases gradually where a new species H4Ce(S04)4 has also been
suggested. ^ Due to complexation in aqueous H2SO4 media, the tendency of
cerium(IV) species to undergo hydrolysis is remarkably suppressed, but in
aqueous HCIO4 media, hydrolysis leads to Ce(OH) ' and Ce(0H)2^^ which
further undergo dimerization producing CeOCe ^.
The cerium(IV) equilibria are important in interpreting oxidation of
organic compounds. Kinetic and additional spectral measurements also indicate
that the degree of eerie perchlorate association depends on acid
concentration. ~ ^ However, a monomer-dimer equilibrium may be an
oversimplification, since above pH 0,7 colloidal polymers slowly form,'' Ceric
nitrate equilibria are complicated by dimerization, hydrolysis, and association
"W '\'y
with cerium(III). ' This could provide an added complexity in cerium(IV)
oxidations where appreciable quantities of cerium(III) are formed.
A good number of studies of the kinetics and mechanism of the oxidation
of a variety of organic substrates have been made by cerium(IV) either in
sulfuric acid or perchloric acid medium. '"' ' Oxidation of carbohydrates by
cerium(IV) has also been a subject of interest, ' ^"" especially to find out if the
same mechanism is operative in this case, too, as it operates in the case of
alcohols, glycols, formaldehyde, etc.
Mehrotra '' ^ investigated the degradation of aldoses by cerium(IV)
sulfate in aqueous sulfuric acid. Pottenger and Johnson^^ studied the mechanism
of cerium(IV) oxidation of glucose and cellulose in 1 mol dm~ perchloric acid.
The oxidation of D-glucose, ' *''*° D-galactose,"*^ L-arabinose,"*^ and L-sorbose'*^
by cerium(IV) has been studied, the reaction generally proceeded to give the
corresponding lactones and aldonic acids.
Sala and coworkers ' ^ reported the oxidative decarboxylation of lactones
by cerium(IV) for the synthesis of 2-deoxy-D-erythro-pentose and D-arabinose.
Virtanen and coworkers'*^ also studied the oxidation of various aldoses and
ketoses by cerium(IV) in perchloric acid and reported the fonnation of two
complexes in each case, one in pre-equilibrium reaction during mixing and other
by the dissociation of first one. Sen Gupta et al.^^ investigated the oxidations of
aldoses like D-ribose, D-erythrose, and DL-glyceraldehyde and compared the
results obtained with that of D-glucose. The results showed that the oxidation of
D-glucose, D-ribose, and D-erythrose by cerium(IV) were kinetically similar and
DL-glyceraldehyde was oxidized by a different mechanism.
10
C. Surfactant and Surfactant Micelles
Interfaces are the boundary regions that separate different bulk regions of
matter. They have special chemical, physical, and biological properties that have
fascinated and drawn the attention of scientists from many different fields.
What makes the interface unique is the asymmetry in forces that is experienced
by molecules and atomic species located there together with the almost two
dimensional geometry of the interface. The chemical composition, the
geometrical arrangement of the species, the equilibrium constants, pH, the
motion of molecules, the thermodynamics and kinetics of ground- and excited-
state chemical change, energy relaxation, and the phases and phase transitions of
long chain amphiphilic monolayers are among the fundamental manifestations of
the unique characteristics of an interface. ^ The study of chemical reactivity at
liquid interfaces occupies an important place in chemistry. ^ Electron transfer,
ion transfer, and proton transfer at the interfaces between two immiscible liquids
are fundamentally important for understanding processes such as liquid
chromatography, phase transfer catalysis, ^ drug delivery problems in
pharmacology, ^ and other phenomena in membrane biophysics. ^ The uptake of
pollutants by water clouds, an important atmospheric phenomenon, * involves
reaction such as ionization at the water liquid/vapor interface.
A surfactant (a contraction of the term surface active agent) is a
substance that when present at low concentration in a system, has the property of
adsorbing onto the surfaces or interfaces of the system and of altering to a
11
marked degree the surface or interfacial free energies of the surfaces (or
interfaces).
Surfactants are compounds whose molecules are fitted with pronounced
lipophilic and hydrophilic moieties; they are amphiphilic molecules. A process
whereby dissolved surfactant molecules react to the repelling action of
surrounding water is aggregation to form various kinds of supramolecular
structures. ^
A wide variety of surfactant compounds can be dispersed in aqueous
solution to form organized assemblies, either by spontaneous combination or
with the aid of sonication. Their formation can be rationalized in terms of
hydrophobic-hydrophilic and electrostatic interactions, '' as well as by
thennodynamic considerations. ' Interest in the biological function of some of
these assemblies and unusual control of reactivity has prompted a number of
structural studies of the different media.
Surfactants find application in almost every chemical industry, including
detergents, paints, dyestuffs, cosmetics, pharmaceuticals, agrochemicals, fibres,
plastics. Moreover, surfactants play a major role in the oil industry, e.g., in
enhanced and tertiary oil recovery. They are also used for environmental
protection, e.g., in oil slick dispersants. Therefore, a fundamental understanding
of the physical chemistry of surface active agents, their unusual properties, and
their phase behavior is essential for most industrial chemists. In addition, an
understanding of the basic phenomena involved in the application of surfactants.
12
such as in the preparation of emulsions and suspensions and their subsequent
stabilization, in microemulsions, in wetting, spreading, and adhesion, etc., is of
vital importance in arriving at the right composition and control of the system
involved. This is particularly the case with many formulations in chemical
industry 7'
Apart from the traditional use of surfactants, surfactant structures are
increasingly being investigated as organic templates to synthesize mesoscopic
inorganic materials with controlled nanoscale porosity, which are expected to
have applications in electronics, optics, magnetism, and catalysis. "*
Surfactants are also utilized in various biochemical methods, such as the
purification and analysis of proteins, in analytical methods based on enzymes or
immunological techniques, and in cleaning and regenerating chromatographic
columns, biosensors, etc. Their ability to hinder protein adsorption is used, both
to reduce the depletion of the substance that should be analyzed due to
adsorption of the walls of test tubes, etc.^^ and to hinder nonspecific adsorption
of proteins in, e.g., immunological methods and chromatography.
General classification of surfactants
Numerous variations are possible within the structure of both the head
and tail group of surfactants. The head group can be charged or neutral, small
and compact in size, or a polymeric chain. The tail group is usually a single or
13
double, straight or branched hydrocarbon chain, but may also be a fluorocarbon,
or siloxane, or contain aromatic group(s).
Since the hydrophilic part normally achieves its solubility either by ionic
interactions or by hydrogen bonding, the simplest classification is based on
surfactant head group type, with further subgroups according to the nature of the
lyophobic moiety. Four basic classes therefore emerge as: the anionics and
cationics (which dissociate in water into two oppositely charged species, i.e., the
surfactant ion and its counterion), the nonionics (include a highly polar (non
charged) moiety, such as polyoxyethylene (—OCH2CHO—) or polyol groups)
and the zwitterionics or amphoterics (combine both as a positive and a negative
group).
Examples:
(anionic)
CH3(CH2)nOS03~ Na
sodium dodecyl sulfate
CH3(CH2)ioCOO~ K
potassium laurate
(cationic)
CH3(CH2),5N (CH3)3Br-
cetyltrimethylammonium-bromide
\ +N-C,2H25Br-
dodecylpyridinium bromide
14
(zwitterionic)
C12H25N^(CH3)2CH2COO"
N-dodecyl-N:N-dimethyl-betaine
CHJCCHS), IN^(CH3)2CH2-CH2-CH2S03"
3 -(dimethy Idodecy lammonio) propane-1-sulfonate
(nonionic)
t-Oct-C6H4-(OCH2CH2)xOH
polyethyleneglycol-t-octyl-phenyl ether (x = 9 or 10)
C9Hi9C6H4(CH2-CH2-0-)n—OH
nonyl phenyl-polyethylene glycol (n~10)
Surfactants are key components of the organized assemblies used in
biological systems. Nonionic surfactants based on carbohydrates are very
important in biology. They have potential pharmaceutical (biocompatible
formulations), biochemical (extraction of membrane proteins), and medicinal
applications. Carbohydrate-based surfactants have water-soluble head group
derived from a carbohydrate. This is linked by different functional groups to a
hydrophobic part.
Surfactants have particular features that make them attractive in relation
to chemical reactivity aspects and for large variety of applications. The
application of surfactant based systems as drug delivery vehicles' ' ^ is a
growing research area that may develop flirther in the coming years. It is quite
interesting that cationic amphiphiles are now widely used as an effective tool in
79,80 delivering DNA into cells, ' even mammalian cells. 81-85
15
Bilayer-forming synthetic surfactants have been extensively used as
membrane mimetic models, and some synthetic amphiphiles such as dihexadecyl
phosphate or dioctadecylmethylammonium salts have found many different uses
in strategic applied areas.*^
Aqueous association colloids as reaction media offer alternatives to the
use of organic solvents, and there is considerable interest in their use in water as
reaction medium; they are attractive candidates in "green" chemistry.* ' ' For
instance, Moss et al.^^'^^ observed 1000- to 2000-fold rate enhancements in
overall rate constants of hydrolysis of phosphates triesters catalyzed by different
iodosocarboxylate ions of varying hydrophobicities in comicelles with CTACl
and CTAOH.
Surfactant based reaction media have such kinds of features that make
them useful in industrial-scale synthesis and interesting in developing "clean"
processes. In fact, they are expected to be nontoxic and nonhazardous, they
enhance reaction rates, reactions can usually be carried out under mild
conditions, and in favorable cases surfactants can be separated and reused.
Surfactants alone or in combination with a wide variety of other ionic and
non-ionic solutes, aggregate spontaneously and with a high degree of
cooperativity in solution to form a variety of assemblies (or association colloids)
whose structures depend both on solution composition and on the structures of
components, primarily the surfactant. ' "*" ^ All surfactant assemblies in
homogeneous solution share an underlying organizational structure: a fluid.
16
hydrocarbon region separated from an aqueous region by an interfacial region
with a thickness of the order of the diameter of the surfactant head group.
Largely depending on the molecular architecture of the amphiphile, a
wealth of three-dimensional structures can be formed ranging from spherical and
rod-like micelles to multilayer structures and to complex biological membranes,
whose matrix is a lipid bilayer composed of phospholipids and glycolipids,
incorporating proteins. ' ' * Ionic colloidal assemblies, e.g., micelles,
microemulsions, hemimicelles (soUoids), bilayers, and vesicles are believed to
be mimetic agents for membranes in biological systems. It has also been noted
that, there are structural similarities between globular proteins and spherical
micelles, and analogies between the catalytic effects of enzymes and functional
micelles and between micellar catalysis and phase-transfer catalysis. ^"' ^ For
these reasons, numerous investigators have focused attention on micelles and
reactions in micellar media.
Micelles, micellar structure and properties
Normal micelles are assemblies of surfactants, which spontaneously
aggregate (micellize) at concentrations greater than critical micelle concentration
(cmc) in water and some associated solvents. Micelles are assumed to be
spherical, with ~10 monomers, at low surfactant concentration, but they grow
at high surfactant concentrations, especially with added electrolyte, and become
rod-like. ' ' The growth depends on the length of the hydrophobic group,
the structure of the head group, and the added electrolyte. Simple rules
17
governing the packing of surfactant in micelles and similar assemblies have been
proposed that relate the geometry of the assembly to the area of the head group
and the length and volume of the hydrophobic residue.^ '"^ The micellar core is
oil like, and ionic or polar head groups at the surface are exposed to water.
Micelles have been investigated by an unusually wide variety of
techniques including X-rays, nuclear magnetic resonance (NMR), electron spin
resonance (ESR), fluorescence, static and dynamic light scattering, calorimetry,
and kinetic probes.''' Micellization is primarily driven by bulk hydrophobic
interactions between the alkyl chains of the surfactant monomers and usually
1 1 ?
results from a favorable entropy change. The overall Gibbs energy of the
aggregate is a compromise of a complex set of interactions, with major
contributions from head group repulsion and counterion binding (for ionic
surfactants). ^ The residence times of individual surfactant molecule in the
micelle are typically of the order of 10" -10~^ s, whereas the lifetime of the
micellar entity is about 10~^-10~'s.
Ever since the discovery of micelles, theoretical models of micelle
formation have in some way tried to account for the association-dissociation
equilibrium that distinguishes micelles from other colloids. The earliest model is
due to Hartley and regards the formation of a micelle as a chemical equilibrium
between monomers, counterions and micelles. This model is the so-called
Hartley micelle and involves a hydrocarbon-like interior surrounded by polar
or ionic head groups. The micelle is pictured as a roughly spherical aggregate
18
with a radius approximately corresponding to the extended length of the
hydrocarbon chain of the surfactant (Fig. 1.1). Micellar head groups and
associated counterions are fully hydrated and are found in the Stern layer. Some
of the counterions are bound within the shear surface, and many are located in
the Gouy-Chapman electrical double layer, where they are dissociated from
micelle. This model has quite appropriately been dubbed the mass action (MA)
approach."'*'"^ The MA approach was followed by the phase separation (PS)
model, advanced by Stainsby and Alexander in 1950,"^ wherein the micelles are
treated as a phase separated from that containing the mesomeric species.
A good model is of Gruen, who has described a realistic model of a
micelle"'''"^ that involves a rather sharp interface between a dry hydrophobic
hydrocarbon core and a region filled with surfactant head groups, some of the
counterions, and water, namely the Stem region. This model has been validated
using molecular dynamic simulations"^''^° and is valid for both ionic and
nonionic micelles.
The overall structure of micelle is characterized by a situation in which
the ionic and polar head groups reside at the surface of the aggregates, where
they are in contact with water, with the alkyl chains in the interior of the micelle
forming a relatively dry hydrophobic core.'^' The alkyl chains of micellized
surfactant are not fiilly extended. Starting from the head group, the first two or
three carbon-carbon bonds are usually trans, whereas gauche conformations are
likely to be encountered near the centre of the chain. As a result, the terminal
19
Aqueous bulk phase
Gouy-Chapman double layer
Fig. 1.1: Model of a typical ionic micelle showing the location of head groups
(©), surfactant chains (AAA) and the counterions (+).
20
methyl moieties of the chain can be located near the surface of the micelle and
may even protrude into the aqueous medium. Consequently, the micellar
surface has a definite degree of hydrophobicity. NMR studies have shown that
the hydrocarbon tails in a micelle are highly mobile and comparable in mobility
to the chains in a liquid hydrocarbon. The degree of water penetration into the
micellar interior has long been a matter of debate. Small-angle neutron
scattering studies have indicated that significant water penetration into the
micellar core is unlikely.'
D. The Pseudophase Model
Kinetic studies in micellar systems have been in the scope of interest of
many researchers for a longtime. The rates of enzymatic, organic, and inorganic
reactions have been investigated in the presence of micelles by a great variety of
surfactants. The chemical literature in the period 1900-1958 contains a
scattering of reports concerning reaction kinedcs in aqueous media containing
ionic or nonionic surfactants. However, substantial insight into this area was first
achieved in 1959 by Duynstee and Grunwald in their study of the effects of
cationic and anionic surfactants on the rate of alkaline fading of cationic
triphenylmethane dyes. Since that dme, related studies have been appearing at
an increasing rate, and interest is still growing.
Association colloids have interfacial regions containing ionic and polar
head groups and ionic and polar solutes may be incorporated in this region.
21
Apolar cores of micelles, which exclude polar and ionic solutes and interfacial
regions, are accessible to these solutes and to water. Consideration of the
dimensions of head groups and apolar tails indicates that the volume of the
interfacial region is approximately half that of the total micelle. Solutions of
dilute surfactants are isotropic, but there is extensive physical evidence that they
are dispersions of submicroscopic particles which form a micellar pseudophase
distinct from the aqueous pseudophase. The interfacial region is highly
anisotropic and in ionic micelles there is considerable neutralization of head
groups by counterions. The intrinsic heterogeneous character of a micellar
solution requires the developments of novel concepts in reaction kinetics since
deviations from conventional rate laws applicable to homogeneous systems are
frequently observed. In particular, the fact has to be taken into account that
compartmentation of the reactants occurs as a consequence of their association
with the surfactant aggregates.' ^
In a homogeneous surfactant solution (above critical micelle
concentration) the reactive site of substrate may exist in one or more of the
following environments: the micelle interior, the micelle water interface, and the
bulk solvent. One of the most important processes leading to the micellar effects
on reactions is the solubilization of substrates in micellar interiors. It is possible
to solubilize water insoluble substances or to increase the solubilities of slightly
soluble ones in aqueous micellar solutions. They penetrate toward the
hydrocarbon-like cores of the micelles.'^^"'" Since the solvent molecules
penetrate beyond the polar head groups, solute in the solvent phase can interact
22
both with the nonpolar chains of surfactant molecules and with polar head
groups. Thus the micellar phase may be referred to as amphipathic, having
affinity for both polar and nonpolar species. Micellar cores behave like an
organic phase and the hydrophobic forces play an important role in the
138
solubilization process.
The pathways and rates of reactions in micelles are affected by how
deeply the solubilized species is located within the micelle. Both electrostatic
and hydrophobic forces play a role in determining the binding site of a solute
inside the micelle, and both the structure of the amphiphile and the solute are of
great importance in determining the extent of solublization and the penetration
of solute into the micelles.
Another fundamental process in micellar catalysis or inhibition is the
counterion binding to micelles. Micelles can either attract the reactive ions or
repel them depending upon the electrical charge of their head groups. Thus,
micelles may bring the solubilized substrates and reactive ions together or keep
them apart such that the reactions are speeded up or inhibited. Another way by
which micelles can catalyze a reaction is the stabilization of intermediates as
bound counterions. ' Sometimes, even substrates are bound to micelles as
their counterions.''^'
Most kinetic treatments are based on so-called pseudophase model. This
model has been generally accepted on the reasonable assumption that for most
activated thermal chemical reactions, transfer of material between water and
23
micelle is so fast that reaction does not perturb the equilibrium distribution of
reactants between the pseudophases. This generalization cannot be applied to
photochemical reactions, where some steps of the reaction may be very rapid
and therefore faster than solute transfer. '*'''' Provided that equilibrium is
maintained between the aqueous and the micellar pseudophases, the overall
reaction rate will be the sum of rates in water and in the micelles and will
therefore depend on the distribution of reactants between each pseudophase and
the appropriate rate constants in the two pseudophases.
Menger and Portnoy'°° developed a quantitative treatment that adequately
described inhibition of ester saponification by anionic micelles. Micelles bound
hydrophobic esters, and anionic micelles excluded hydroxide ions and so
inhibited the reaction, whereas cationic micelles speeded saponification by
attracting hydroxide ions.''*'' Provided that only substrate distribution has to be
considered, which is the situation for micelle-inhibited bimolecular or
spontaneous unimolecular reactions, Scheme 1.3 shows the substrate distribution
and reaction in each pseudophase 144
Dn + S^
k' w
Ke
Products
m
m
Scheme 1.3
24
In the Scheme 1.3, Dn denotes micellized surfactant, S is substrate,
subscripts w and m denote aqueous and micellar pseudophases, respectively, and
k\v and k'm are first-order rate constants. The binding constant, Ks, is written in
terms of the molarity of micellized surfactant, but it could equally be written in
terms of the molarity of micelles. The concentration of micellized surfactant is
that of total surfactant less that of monomer, which is assumed to be given by the
cmc.
The experimental rate constant kvp for Scheme 1.3 is a weighted sum of
the two constants k'w and k'm
k4 = fw k'w + fm k'm = fw k'w + (1 -fw) k'm (1.6)
where fw and fm are the fractions of substrate in the bulk solution and solubilized
in the micelles, respectively. The second version of Eq. (1.6) arises from the
recognition that fw + fm = 1- The equilibrium constant, Kj, in Scheme 1.3 can be
written as
[S„][D„] [D„]f„[S,] [D„](l-fJ ^ '
where [St] is the total substrate concentration.
From Eq. (1.7)
fw = (l+K3[DnF' (1.8)
which, on substitution in Eq. (1.6), gives
25
K w "I" k m K s [ D n ]
kvp = (1.9) 1 +Ks[Dn]
This equation is similar in form to the Michaelis-Menten equation of
enzyme kinetics, although the analogy is limited because most enzymatic
reactions are studied with substrate in large excess over enzyme. Equation (1.9)
could be rearranged to give Eq. (1.10), which is formally similar to the
Lineweaver-Burk equation and which permits calculation of k'm and Ks provided
thatk'wisknown.'°°''^^
1 1 1 + (1.10)
(k'w-ks^) (k'w-k'n.) (k'w-k'm)Ks[Dn]
Equations (1.9) and (1.10) have been applied successfully to micellar
catalyzed unimolecular reactions and to many micellar mediated reactions. The
observations suggest that the pseudophase model is useful in analyzing micellar
catalysis and inhibition. These equations, however, depend on some major
assumptions, in particular that the cmc gives the concentration of monomeric
surfactant and the rate and binding constants in the micellar pseudophase are
unaffected by reactants and products.
E. Statement of the Problem
Cerium(IV) as an oxidant has been employed both in
mechanistic'^'-^"'^•"'^"'^^ as well as synthetic'^°~'" studies despite the fact the
speciation of sulfato-cerium(IV) species is still not established
26
conclusively' •'"' ^ in sulfuric acid medium. Cerium(III) is the only reduction
product of cerium(IV) as the latter is one of a group of metal ion oxidants which
apparently react only via one-electron steps. Cerium(III) has been found usefiil
for the plants as well as animals.'^
Due to very important role of cerium(IV) (as an oxidant), the oxidation of
carbohydrates by cerium(IV) has received attention for a long time, ' " ^ but the
same in presence of surfactants has not been studied so far.
Effect of organized structures (e.g., micelles being one of them) on the
rate of electron transfer reactions has been receiving considerable attention
too. A number of interfacial electron transfer processes have been
investigated in polyelectrolytes, vesicles and micellar surfaces including
photoredox reactions. The interest in this subject arises from the similarity with
the biological processes. Therefore, the present work was undertaken to study
the effect of surfactant micelles on the kinetics and mechanism of oxidation of
carbohydrates by cerium(IV). For this purpose two aldopentoses (D(+)xylose
and L(+)arabinose), two aldohexoses (D(+)glucose and D(+)mannose), and two
ketohexoses (D(-)fructose and L(-)sorbose) were used. The surfactants used in
the study were cationic cetyltrimethylammonium bromide (CTAB) and anionic
sodium dodecyl sulfate (SDS).
27
References
1. ''Carbohydrate Chemistry", J. F. Kennedy (Ed.), Clarendon Press,
Oxford, 1988.
2. "-Molecular Glycobiology", M. Fukuda, O. Hindsgaul (Eds.), Oxford
University Press, Oxford, 1994.
3. J. F. Robyt, ''Essentials of Carbohydrate Chemistry", Springer, New
York, 1997.
4. "Glycosciences: Status and Perspectives", H.-J. Gabius, S. Gabius
(Eds.), Chapman & Hall, Weinheim, 1997
5. J. Rini, K. Drickamer, Curr. Opin. Struct. Biol, 1997, 7, 615.
6. B. M. Pinto, In "Comprehensive Natural Products Chemistry", Vol. 3,
D. Barton, K. Nakanishi (Eds.), Elsevier, New York, 1999.
7. T. Feizi, D. R. Bundle, Curr. Opin. Struct. Biol, 1999, 6, 659.
8. "Bioorganic Chemistry: Carbohydrates", S. M. Hecht (Ed.), Oxford
University Press, New York, 1999.
9. L. F. Sala, A. F. Cirelli, R. M. de Lederkremer, J. Chem. Soc, Perkin
Trans. 2, 1977, 685.
10. J. Barek, A. Berka, A. Pokoma-Hladikova, Collect. Czech. Chem.
Commun., 1982,47,2466.
11. M. Gupta, S. K. Saha, P. Banerjee, J. Chem. Soc, Perkin Trans. 2,
1988, 1781.
28
12. S. Signorella, L. Ciullo, R. Lafarga, L. F. Sala, New J. Chem., 1996, 20,
989.
13. S. Angyal, Adv. Carbohydr. Chem. Biochem., 1989, 47, 1.
14. L. F. Sala, A. F. Cirelli, R. de Lederkremer, Anal. Asoc. Quim. Arg.,
1978,66,57.
15. Z. Li, Y. Yang, J. Liu, J. Pan, J. Tang, Analytical Letters, 2002, 35,
1959.
16. W. H. Richardson, In ''Oxidation in Organic Chemistry, Part A, K. B.
Wiberg (Ed.), Academic Press, New York, 1965.
17. G. E. Smith, C. A. Getz, Ind Eng. Chem. Anal. Ed., 1938, 10, 191.
18. M. S. Sherril, C. G. King, R. C. Spooner, J. Am. Chem. Soc. 1943, 65,
170.
19. F. B. Baker, T. W. Newton, M. Kahn, J. Phys. Chem., 1960, 64, 109.
20. A. A. Noyes, C. S. Gamer, J. Am. Chem. Soc, 1936, 58, 1265.
21. A. H. Kunz, J. Am. Chem. Soc, 1931, 53, 98.
22. E. Wadsworth, F. R. Duke, C. A. Goetz, Anal. Chem., 1957, 29, 7824.
23. T. J. Hardwick, E. Robertson, Can. J. Chem., 1951, 29, 828.
24. L. T. Bugaenko, H. Kuan-Lin, Russ. J. Inorg. Chem., 1963, 8, 1299.
25. E. G. Jones, F. G. Soper, J. Chem. Soc, 1935, 802.
26. T. J. Hardwick, E. Robertson, Can. J. Chem., 1951, 29, 818.
27. L. J. Heidt, M. E. Smith, J. Am. Chem. Soc, 1948, 70, 2476.
28. F. L. King, M. Pandow, J. Am. Chem. Soc, 1952, 74, 1966.
29. M. Ardon, J. Chem. Soc, 1957, 1811.
29
30. M. Ardon, G. Stein, J. Chem. Soc, 1956, 104.
31. B. D. Blaustein, J. W. Gryder, J. Am. Chem. Soc, 1957, 79, 540.
32. M. K. Dorfman, J. W. Gryder, Inorg. Chem., 1962,1, 799.
33. T.-L. Ho, Synthesis, 1973, 347.
34. A. K. Das, Coord. Chem. Rev., 2001, 213, 307.
35. R. N. Mehrotra, Z phys. Chem., 1965, 230, 221.
36. C. R. Pottenger, D. C. Johnson, J. Polym. Sci: Part A-U 1970, 8, 301.
37. M. G. R. Reddy, B. Sethuram, T. N. Rao, Curr. Sci., 1973, 42, 677.
38. R. N. Mehrotra, E. S. Emis, J. Org. Chem., 1974, 39, 1788.
39. R. M. de Lederiaemer, L. F. Sala, Carbohydr. Res., 1975, 40, 385.
40. V. I. Krupenskii, Zh. Obshch. Khim., 1978, 48, 2228.
41. Y. R. Rao, P. K. Saiprakash, Curr. Sci., 1978, 47, 763.
42. V. I. Krupenskii, Zh. Obshch. Khim., 1979, 49, 457.
43. A. Kale, K. C. Nand, Gazz. Chim. Ital, 1982,112, 396.
44. A. Kale, K. C. Nand, Z phys. Chem. (Leipzig), 1983, 264, 1023.
45. P. Singh, R. Singh, A. K. Singh, E. B. Singh, J. Indian Chem. Soc,
1985,62,206.
46. A. G. Fadnis, Carbohydr. Res., 1986, 146, 97, and the references cited
therein.
47. P. O. I. Virtanen, R. Lindroos, E. Oikarinen, J. Vaskuri, Carbohydr.
Res., 1987,167,29.
48. P. O. I. Virtanen, R. Lindroos-Heinanen, Acta Chem. Scand., 1988, B
42,411.
30
49. K. K. Sen Gupta, S. Sen Gupta, A. Mahapatra, J. Carbohydr. Chem.,
1989,8,713.
50. K. K. Sen Gupta, S. Sen Gupta, S. K. Mandal, A. Mahapatra, J. Chem.
Res. (S;, 1990, 60.
51. M. P. Sah, J. Indian Chem. Soc, 1995, 72, 173.
52. A. Roy, A. K. Das, Indian J. Chem., 2002, 41A, 2468.
53. A. Agarwal, G. Sharma, C. L. Khandelwal, P. D. Sharma, Inorg. React.
Mech., 2002, 4, 223.
54. A. W. Adamson, ''Physical Chemistry of Surfaces'", 4"" ed., Wiley, New
York, 1982.
55. K. B. Eisenthal, Ace. Chem. Res., 1993, 26, 636.
56. I. Benjamin, Chem. Rev., 1996, 96, 1449.
57. C. M. Starks, C. L. Liota, M. Halpern, ''•Phase Transfer Catalysis'',
Chapman & Hall, New York, 1994.
58. K. Arai, M. Ohsava, F. Kusu, K. Takamura, Bioelectrochem. Bioenerg.,
1993,31,65.
59. R. B. Gennis, "Biomembranes", Springer, New York, 1989.
60. "The Chemistry of Acid Rain: Sources and Atmospheric Processes", R.
W. Johnson, G. E. Gordon (Eds.), ACS Symposium Series, American
Chemical Society, Washington, D C, 1987.
61. M. J. Rosen, "Surfactants and Interfacial Phenomena", 2"'' ed., Wiley,
New York, 1978.
31
62. G. Savelli, R. Germani, L. Brinchi, In ''Reactions and Synthesis in
Surfactant Systems'', John Texter (Ed.), Marcel Dekker, New York,
2001.
63. C. Tanford, ''The Hydrophobic Effect: Formation of Micelles and
Biological Membranes'", 2" * ed., Wiley-Interscience, New York, 1980.
64. C. Tanford, In "Micellization, Solubilization and Microemulsions",
Vol. 1, K. L. Mittal (Ed.), Plenum Press, New York, 1979.
65. K. A. Dill, P. J. Flory, Proc. Natl. Acad ScL, U.SA.,\9m, 78, 676.
66. F. M. Menger, J. M. Bonicap, J. Am. Chem. Soc, 1981, 103, 2140.
67. A. S. Waggoner, O. H. Griffith, C. R. Christennes, Proc. Natl. Acad.
ScL, a s A., 1967,57, 1198.
68. E. .1. Fendler, C. L. Day, J. H. Fendler, J. Phys. Chem., 1972, 76, 1460.
69. M. Gratzel, J. K. Thomas, J. Am. Chem. Soc, 1973, 95, 6885.
70. R. Breslow, S. Kitabataki, T. Rothhard, J. Am. Chem. Soc, 1978, 100,
8156.
71. S. S. Atik, L. A. Singer, Chem. Phys. Lett., 1978, 59, 519.
72. M. F. Czamiecki, R. Breslow, J. Am. Chem. Soc, 1979, 101, 3675.
73. T. F. Tadros, "Applied Surfactants: Principles and Applications",
Wiley-VCH, Weinheim, 2005.
74. N. Kimizuka, T. Kunitake, Adv. Mater., 1996, 8, 89.
75. B. Warren, S. P. Kusk, R. G. Wolford, J. Biol. Chem., 1996, 271,
11434.
76. L. J. J. Hronowski, T. P. Anastassiades, Anal. Biochem., 1990, 191, 50.
32
77. D. W. Osborne, A. J. Ward, K. J. O'Neill, J. Pharm. Pharmacol, 1991,
43,451.
78. J. Kemken, A. Ziegler, B. W. Muller, Pharm. Res., 1992, 9, 554.
79. R. Leventis, J. R. Silvius, Biochim. Biophys. Acta, 1990, 1023, 124.
80. T. Akao, T. Osaki, J. Mitoma, A. Ito, T. Kunitake, Bull. Chem. Soc.
Jpn., 1991,64,3677.
81. P. L. Feigner, T. R. Gadek, M. Holm, R. Ronas, H. W. Chan, M. Wenz,
J. P. Northrop, G. M. Ringold, M. Danielsen, Proc. Natl. Acad. ScL,
a 5 . A, 1987,84,7413.
82. P. Pinnaduwage, L. Schmitt, L. Huang, Biochim. Biophys. Acta, 1989,
985,33.
83. J. Y. Legendre, F. C. Szoka, Proc. Natl. Acad. ScL, U. S. A., 1993, 90,
893.
84. B. J. Roessler, B. L. Davidson, Neurosci. Lett., 1994,167, 5.
85. Y. Watanabe, H. Nomoto, R. Takezava, N, Miyoshi, T. Akaike, J.
Biochem., 1994,116, 1220.
86. A. M. Carmona-Ribeiro, Chem. Soc. Rev., 1992, 209.
87. R. Schomacher, J. Chem. Res. (S), 1991, 92.
88. K. Holmberg, Adv. Colloid Interface Scl, 1994, 51, 137.
89. F. M. Menger, H. Park, Rec. Trav. Chim. Pays-Bas, 1994, 113, 176.
90. S. G. Oh, J. Kizling, K. Holmberg, Colloids Surf. A: Physicochem. Eng.
Aspects, 1995, 97, 167.
33
91. M. J. Schwuger, K. Stickdom, R. Schomacker, Chem. Rev., 1995, 95,
849.
92. R. A. Moss, A. T. Kotchever, B. D. Park, P. Scrimin, Langmuir, 1996,
12, 2200.
93. R. A. Moss, S. Base, Tetrahedron Lett, 1997, 38, 965.
94. J. H. Fendler, ''Membrane Mimetic Chemistry", Wiley- Interscience,
New York, 1982.
95. J. N. Israelachvili, ''Intermolecular and Surface Forces'', 2"'' ed.,
Academic Press, London, 1991.
96. ''Handbook of Surface and Colloid Chemistry'', K. S. Birdi (Ed.), CRC
Press, Boca Raton, FL, 1997.
97. F. Vogtle, "Supramolecular Chemistry", Wiley, Chichester, U. K.,
1991.
98. J. H. Fuhrhop, J. Koning, "Membranes and Molecular Assemblies: The
Synkinetic Approach", The Royal Society of Chemistry, London, 1994.
99. M. T. A. Behme, J. Fullington, R. Noel, E. H. Cordes, J. Am. Chem.
Soc, 1965,87,266.
100. F. M. Menger, C. E. Portnoy, J. Am. Chem. Soc, 1967, 89, 4698.
101. T. E. Wagner, C. Hsu, C. S. Pratt, J. Am. Chem. Soc, 1967, 89, 6366.
102. M. L. Bender, T. H. Marshall, J. Am. Chem. Soc, 1968, 90, 201.
103. R. B. Dunlap, E. H. Cordes, J. Am. Chem. Soc, 1968, 90, 4395.
104. L. S. Romsted, E. H. Cordes, J. Am. Chem. Soc, 1968, 90, 4404.
34
105. J. Baumrucker, M. Calzadilla, M. Centeno, G. Lehrmann, M. Urdaneta,
P. Lindquist, D. Dunham, M. Price, B. Sears, E. H. Cordes, J. Am.
Chem.Soc, 1972,94,8164.
106. J. H. Fendler, E. J. Fendler, ''Catalysis in Micellar and
Macromolecular Systems", Academic Press, New York, 1975.
107. F. Nome, A. F. Rubira, C. Franco, L. G. lonescu, J. Phys. Chem., 1982,
86, 1881.
108. Z. Djeghaba, H. Deleuze, B. De Jeso, D. Messadi, B. Maillard,
Tetrahedron Lett., 1991, 32, 761.
109. ''Surfactant Solutions: New Methods of Investigation", R. Zana (Ed.),
Marcel Dekker, New York, 1987.
110. J. N. Israelachvili, D. J. Mitchell, B. W. Ninham, J. Chem. Soc,
Faraday Trans. 2, 1976, 72, 1525.
111. S. Tascioglu, Tetrahedron, 1996, 52, 11113.
112. W. Blokzijl, J. B. F. N. Engberts, Angew. Chem. Int. Ed. Engl, 1993,
32, 1545.
113. G. S. Hartley, R. C. Murray, Trans. Faraday Soc, 1935, 31, 183.
114. P. H. Elworthy, K. J. Mysels, J. Colloid Interface Scl, 1966, 21, 331.
115. P. Mukerjee, Adv. Colloid Interface Scl, 1967, 1, 241.
116. G. Stainsby, A. E. Alexander, Trans Faraday Soc, 1950, 46, 587.
117. D. W. R. Gruen, J. Colloid Interface Scl, 1981, 84, 281.
118. D. W. R. Gruen, E. H. B. deLacey, In "Surfactants in Solution", Vol. 1,
K. L. Mittal, B. Lindman (Eds.), Plenum Press, New York, 1984.
35
119. J. Shelley, K. Watanabe, M. L. Klein, Int. J. Quantum Chem. Quantum
Biol. Symp., 1990, 17, 103.
120. J. Bocher, J. Brickmann, P. Bopp, J. Phys. Chem., 1994, 98, 712.
121. D. W. R. Gruen, Prog. Colloid Polym. Sci., 1985, 70, 6.
122. J. Clifford, Trans. Faraday Soc, 1965, 61, 1276.
123. H. Walderhaug, O. Soderman, P. Stilbs, J. Phys. Chem., 1984, 88,
1655.
124. S. S. Berr, E. Caponetti, J. S. J. Johnson, R. R. M. Jones, L. S. Magid,
J. Phys. Chem. 1986, 90, 5766.
125. E. F. J. Duynstee, F. Grunwald, J. Am. Chem. Soc, 1959, 81, 4540.
126. R. C. Dorrence, T. F. Hunter, J. Chem. Soc, Faraday Trans. 1, 1972,
68,1312.
127. P. P. Infelta, M. Gratzel, J. K. Thomas, J. Phys. Chem., 1974, 78, 190.
128. Y. Waka, K. Hamamoto, N. Malaga, Chem. Phys. Lett., 1978, 53, 242.
129.. J. W. Conine, J. Pharm. Sci., 1965, 54, 1581.
130. K. H. Kee, P. de Mayo, J. Chem. Soc, Chem. Commun., 1979, 493;
Photochem. Photobiol, 1980, 31, 311.
131. A. Seret, A. Van de Vorst, J. Phys. Chem., 1990, 94, 5293.
132. S. Mazumdar, J. Phys. Chem., 1990, 94, 5947.
133. P. Baglioni, E. Rivara-Minten, L. Dei, E. Ferroni, J. Phys. Chem. 1990,
94.8218.
134. Y. Kubota, N. Omura, K. Murakami, Bull. Chem. Soc. Jpn. 1991, 64,
814.
36
135. E. B. Abuin, E. A. Lissi, J. Chem. Educ, 1992, 69, 340.
136. F. M. Menger, C. E. Mounier, J. Am. Chem. Soc, 1993, 115, 12222.
137. Z. Lin, J. J. Cai, L. E. Scriven, H. T. Davis, J. Phys. Chem., 1994, 98,
5984.
138. K. Kano, S. Tatemoto, S. Hashimoto, J. Phys. Chem., 1991, 95, 966.
139. E. J. Fendler, J. H. Fendler, Adv. Phys. Org. Chem., 1970, 8, 271.
140. X. Li, G. Zhao, Colloids Surf., 1992, 64, 185.
141. S. Harada, H. Okada, T. Sano, T. Yamashita, H. Yano, J. Phys. Chem.,
1990,94,7648.
142. ""Chemistry of Excitation at Interfaces'", J. K. Thomas (Ed.), ACS
Monograph 184, American Chemical Society, Washington, D C, 1984.
143. F. M. Menger, Pure Appl. Chem., 1979, 51, 999.
144. C. A. Bunton, L. Robinson, J. Am. Chem. Soc, 1968, 90, 5972.
145. H. H. Willard, P. Young, J. Am. Chem. Soc, 1930, 52, 132.
146. J. Shorter, J. Chem. Soc, 1950, 3425.
147. F. R. Duke, R. F. Bremer, J. Am. Chem. Soc, 1951, 73, 5179.
148. G. Hargraves, L. H. Sutcliffe, Trans. Faraday Soc, 1955, 51, 1105.
149. W. A. Waters, J. R. Jones, J. S. Littler, J. Chem. Soc, 1961, 240.
150. S. S. Muhammad, K. V. Rao, Bull. Chem. Soc Jpn., 1963, 36, 949.
151. H. L. Hintz, D. C. Johnson, J. Org. Chem., 1967, 32, 556.
152. V. K. Grover, Y. K. Gupta, J. Inorg Nucl. Chem., 1969, 31, 1403.
153. C. F. Wells, M. Hussain, Trans. Faraday Soc, 1970, 66, 679.
37
154. T. R. Balasubramanian, N. Venkatasubramanian, Indian J. Chem.,
1970,8,305.
155. C. R. Rao, Indian J. Chem., 1970, 8, 328.
156. R. Dayal, G. V. Bakore, Indian J. Chem., 1972, 10, 1165.
157. P. K. Saiprakash, B. ^dhmmi, Indian J. Chem., 1973,11, 246.
158. J. Manuel, G. Meza, M. Spiro, Inorg. Chem., 1991, 84, 53.
159. F. Mata Perez, C. Francesco, N. P. Alvarez, Z anorg. allg. Chem.,
2002,628,431.
160. S. K. Mishra, P. D. Sharma, Y. K. Gupta, J. Inorg. Nucl. Chem., 1974,
36, 1845.
161. S. A. Dikshitulu, V. H. Rao, S. N. Dindi, Indian J. Chem., 1980, 19A,
203.
162. S. N. Tanveer, S. T. Nandibewoor, J. R. Raju, Indian J. Chem., 1991,
29A, 92.
163. A. McAuley, Coord Chem. Rev., 1970, 5, 245.
164. G. Galliani, B. B. Rindone, C. Scolastico, Synth. Commun., 1975, 5,
319.
165. V. Devra, I. Sharma, P. D. Sharma, Int. J. Chem. Kinet., 1993, 25, 538.
166. C. A. Bunton, G. Cerichelli, Int. J. Chem. Kinet., 1980, 12, 519.
167. F. P. Cavasino, C. Sbriziolo, E. Pelizzetti, Ber. Bunsenges. Phys.
Chem., 1983,87,843.
168. C. Minero, E. Pramauro, E. Pelizzetti, D. Meisel, J. Phys. Chem., 1987,
83, 399.
39
A. Materials
Table 2.1 summarizes the chemicals which were used throughout the
study. The surfactants, cetyltrimethylammonium bromide (CTAB) and sodium
dodecyl sulfate (SDS), as well as all other chemicals were used as such without
further purification.
B. Preparation of Solutions
The solutions were prepared by directly dissolving the weighed samples
in doubly-distilled water (distillation carried out over alkaline KMn04 in an all-
Pyrex distillation set-up) having specific conductivity 1-2 x 10~ S cm" . The
compound ammonium eerie nitrate is available as a primary standard, and
standard solutions were made by direct weighing followed by dilution in a
volumetric flask. Fresh solutions of carbohydrates were prepared before use.
C. Kinetic Measurements
The kinetic experiments were carried out at constant temperatures
controlled within ±0.1 °C in a thermostat which was designed and assembled in
the laboratory with commercially available components. Known volumes of all
the reactants (except reductant, i.e., monosaccharide) were taken in a three-
necked reaction vessel equipped with a double-surface water condenser to
prevent evaporation. The aqueous solution of reductant was taken in another
flask. Both containers were then placed in the thermostat at the desired
40
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43
temperature for sufficient time to attain thermal equilibrium. Required volume of
the reductant was then added to other reactants (cerium(IV), H2SO4, etc.) and
zero time noted when half of the monosaccharide solution had been added.
Monosaccharide was always in excess {ca. ten times) of cerium(IV) solution in
order to maintain pseudo-first-order conditions. The progress of the reaction
was followed at 385 nm by monitoring the absorbance changes of cerium(IV)
(cerium(III) is transparent at this wavelength) ' with a Bausch and Lomb
Spectronic-20 spectrophotometer. For this purpose, aliquots of the reaction
mixture were taken out at definite time intervals and cooled in an ice bath to
quench the reaction before each measurement. The pseudo-first-order rate
constants in absence (kobs> s~') and presence of surfactants (k /, s~') were
calculated from the linear parts of the plots of log(absorbance) versus time by
carrying out reactions up to ~80% completion.
The dependence of kobs or kp was obtained as a function of [oxidant],
[H2SO4], [reductant], [804^1, [HS04~], [surfactant], temperature and [salt]. The
results are given in Chapter 3.
D. Determination of cmc
(a) by conductivity measurements
Aqueous solutions of surfactants show normal electrical conductivities
and follow the Onsager equation at very low concentrations. Deviations from
Onsager plots occur after critical concentrations of the surfactants. Above the
44
critical concentration a sharp fall in conductance is generally observed. This is
interpreted as caused by micelle formation owing to the presence of highly
charged ions above the critical concentration.
It is well known^ that the specific conductivity is linearly related to the
surfactant concentration in both the pre-micellar as well as in the post-micellar
regions, the slope in the pre-micellar region being greater than that in the post-
micellar region. The intersection point between the two straight lines gives the
cmc.
The conductivity measurements were made using an ELICO (Hyderabad,
India) bridge (type CM 82T) with platinized electrodes (cell constant = 1.02
cm"'). The conductivity of the solvent (taken in a glass container which was kept
in a thermostat at the desired temperature) was measured before the addition of
CTAB. Small amounts of CTAB were added successively, mixed well and the
conductivity was recorded. The specific conductivity was calculated applying
solvent corrections. The cmc values obtained under different conditions are
listed in Table 2.2.
(b) by surface tension measurements
Due to high concentration of H2SO4 (1.83 mol dm~ ) in our experimental
conditions, it was not possible to measure the cmc of CTAB conductimetrically
in presence of the acid. Therefore, the surface tension measurements were also
performed for obtaining the cmc in presence of H2SO4.
45
TABLE 2.2:
Critical micelle concentration (cmc) values of CTAB in different solutions at
40 °C (by conductimetric method).
Solution 10'*cmc(moldm )
Water 10.2(9.8,11.5)'
Water + cerium(IV) 10.0
Water + D(+)xy lose 10.0
Water + L(+)arabinose 10.0
Water + D(+)glucose 10.0
Water + D(+)mannose 9.9
Water + D(-)fructose 9.9
Water + L(-)sorbose 9.5
Water + cerium(lV) + D(+)xylose 8.3
Water + cerium(IV) + L(+)arabinose 8.8
Water + cerium(IV) + D(+)glucose 10.0
Water + cerium(IV) + D(+)mannose 9.8
Water + cerium(IV) + D(-)fructose 9.9
Water + cerium(IV) + L(-)sorbose 9.4
'[Ce(IV)]T = 1.0 X 10' mol dm~^ [carbohydrate]! = 4.0 x 10" mol dm"^
''The literature values at different temperatures (35 and 45 °C) by conductance
measurement are quoted in the parenthesis.^
46
Surface tension is probably the most common means of determining the
cmc. Below the cmc, surfactant molecules position themselves at the airWater
interface and thus lower the surface tension. Above the cmc, any added
surfactant monomer prefers to join micelle rather than to enter the interfacial
film. Consequently, a plot of surface tension versus concentration decreases
steeply below the cmc and levels off above it, the break in the plot is taken as the
cmc. The method is fast, convenient, and non-destructive to the surfactant.
The surface tension was measured by the ring detachment method using a
S.D. Hardson tensiometer (Kolkata, India). The addition of surfactant solution to
the solvent was similar as described under conductivity measurements. The cmc
values of CTAB obtained under different experimental conditions are given in
Table 2.3.
We see remarkable changes in the aggregation behavior of CTAB in
presence of H2SO4. These results of cmc decrease are in accord with the earlier
observations of micelle formation at relatively low surfactant concentration with
the rise of water content.^"'°
E. Product Analysis
(a) characterization of ceriuni(III)
To identify the reaction product of cerium(IV) present in the reaction
mixture, several kinetic experiments were performed taking [Ce(IV)]T = 1.0 x
10' mol dm"^ [reductant]T = 4.0 x 10~^mol dm' and [H2SO4] = 1.83 mol dm'
47
TABLE 2.3:
Critical micelle concentration (cmc) values of CTAB in different solutions at
40 °C (by surface tension measurements).^
T Solution 10 cmc(moldm )
Water
Water + cerium(IV)
Water + H2SO4
Water + D(+)xylose
Water + L(+)arabinase
Water + D(+)glucose
Water + D(+)mannose
Water + D(-)fructose
Water + L(-)sorbose
Water + cerium(IV)+ H2SO4 + D(+)xylose
Water + cerium(IV)+ H2SO4 + L(+)arabinose
Water + cerium(IV)+ H2SO4 + D(+)glucose
Water + cerium(rV)+ H2SO4 + D(+)mannose
Water + cerium(IV)+ H2SO4 + D(-)fructose
Water + cerium(IV)+ H2SO4 + L(-)sorbose
9.9 (8.0f
2.0
0.21
8.3
9.1
9.9
8.3
9.1
9.1
0.20
0.15
0.16
0.13
0.16
0.15
'[Ce(IV)]T = 1.0 X 10" mol dm"\ [carbohydrate]! = 4.0 x 10" mol dm'\
[H2S04]T=1.83moldm~^
The literature value at 25 °C by surface tension measurement.^
48
at 40 °C. The spectra of the mixtures were recorded at different time intervals,
which show that, as the reactions progress, the peak at 385 nm (which was
observed at zero reaction time) decreases. At the end of the reactions, under the
above experimental kinetic conditions, the cerium(IV) peak was found absent
but appearance of any new peak was not observed as well (for example, see
Fig. 2.1, A, for Ce(IV)-fructose reaction). These spectral studies show that the
reaction product formed in the reaction of cerium(IV) and carbohydrates is
transparent. This is the indication of the presence of cerium(III)' as one of the
reaction products.
Same behavior of the spectra was observed in presence of CTAB (Fig.
2.1, B) indicating that cerium(III) is formed in CTAB medium also.
(b) lactones and aldonic acids
Qualitative analyses of the oxidized reaction mixtures with the excess
[reductant] over [Ce(IV)] (the kinetic condition) in presence of H2SO4 were
performed for each carbohydrate. After the kinetic experiment was over, a part
of the oxidized reaction mixture was treated with alkaline hydroxylamine
solution, and the presence of lactone in the reaction mixture was tested by FeCls
— HClbluetest."''^
To the other part of the reaction mixture, barium carbonate was added to
make the solution neutral.'^ FeCls solution that had been colored violet with
phenol when added to this reaction mixture gave a bright-yellow coloration,''*
indicating the presence of aldonic acid.
49
a* u c o X)
o
<
360 400 AAO
WQvelength(nrn)
A80
Fig. 2.1: Absorption spectra of the reaction product of cerium(IV) and
D(-)fructose in absence (A) and presence (B) of CTAB at 40 °C:
( • ) immediately after mixing the reactants, [Ce(IV)]x = 1.0 x 10"
mol drn'^ [D(-)fructose]T = 4.0 x 10" mol dm~ and [H2S04]T =
1.83 mol dm'- ; (©) after 30 min and (O) after 120 min.
50
(c) free radical detection
Acrylonitrile monomer was used to test the presence of free radicals in
the reaction mixtures. Appearance of a precipitate of white polymeric product
was observed when monomer solution (30%) was added to reaction mixtures
containing [Ce(IV)]T = 1.0 x 10" mol dm"\ [reductantji = 4.0 x 10" mol dm"
and [H2S04]T = 1-83 mol dm~ . This confirms the formation of free radicals
during the oxidation of carbohydrates by cerium(IV).
(d) stoichiometry
To determine the oxidant : reductant stoichiometric ratios, a series of
kinetic runs with a fixed concentration of cerium(IV) (= 1.0 x 10" mol dm~ ) in
excess and different concentrations of reductant in presence of 1.83 mol dm"
H2SO4 were performed. After completion of the reactions, the unconsumed
oxidant was estimated spectrophotometrically. The consumption ratios, i.e., the
number of moles of cerium(IV) consumed per mole of carbohydrate (calculated
by assuming that the carbohydrates were totally consumed under these
conditions) were found to be (cerium(IV) : carbohydrate) 6 : 1,4: 1,4: 1,4: 1,
5 : 1, 3 : 1, for D(+)xylose, L(+)arabinose, D(+)glucose, D(+)mannose,
D(-)fructose and L(-)sorbose, respectively. Due to autoaccelaration nature of
the reaction iyide infra), the exact stoichiometry equation and products formed
are difficult to predict. However, the carbohydrates have been used in sufficient
excess throughout the kinetic investigations to ensure that the rate of reduction
of the cerium(IV) is proportional to the rate of oxidation of organic substrates
51
themselves, but not to the rate of destruction of any reactive organic
intermediates,'^ Moreover, since the initial rate of consumption of cerium(IV)
under these conditions was always first-order, the rate of oxidation of the
intermediate products cannot be kinetically significant.'
52
References
1. R. A. Day, Jr., A. L. Underwood, ''Quantitative Analysis", 6* ed..
Prentice Hall, Englewood Cliffs, N J, U. S. A., 1991.
2. G. Hargraves, L. H. Sutcliffe, Trans. Faraday Soc, 1955, 51, 1105.
3. Z. Khan, Raju, Kabir-ud-Din, Colloids Surf. A: Physicochem. Eng.
Aspects, 2003, 225, 75.
4. G. S. Hartley, Trans. Faraday Soc, 1935, 31, 31.
5. R. Zana, J. Colloid Interface Sci., 1980, 78, 330.
6. P. Mukerjee, K. J. Mysels, ''Critical Micelle Concentrations of
Aqueous Surfactant Systems", NSRDS-NBS 36, Superintendent of
Documents, Washington, DC, 1971.
7. A. Muller, S. Giersberg, Colloids Surf, 1992, 69, 5.
8. J. Steigman, N. Shane, J. Phys. Chem., 1965, 69, 968.
9. F. M. Menger, J. M. Jerkunica, J. Am. Chem. Soc, 1979, 101, 1896.
10. B. E. Gillespie, M. J. Smith, P. A. H. Wyatt, J. Chem. Soc, 1969, 1896.
11. K. K. Sen Gupta, B. A. Begum, B. B. Pal, Carbohydr. Res., 1998, 309,
303.
12. M. Abdel-Akher, F. Smith, J. Am. Chem. Soc, 1951, 73, 5859.
13. R. N. Mehrotra, E. S. Emis, J. Org. Chem., 1974, 39, 1788.
14. K. K. Sen Gupta, B. A. Begum, B. B. Pal, Carbohydr. Res., 1999, 315,
70.
15. P. A. Best, J. S. Littler, W. A. Waters, J. Chem. Soc, 1962, 822.
54
A. Results
In aqueous H2SO4 media, cerium(IV) is both thermodynamically and
kinetically weaker as an oxidizing agent. However, due to greater stability''^ and
not requiring any special precaution to prevent its photochemical decomposition^
(which occurs spontaneously in aqueous HCIO4), cerium(IV) in aqueous H2SO4
media is very often used as an oxidant. The present work, therefore, deals with
the redox reactions of cerium(IV) and carbohydrates carried out in H2SO4 media
in the absence and presence of surfactants.
The method of kinetic measurements has already been described in
Chapter 2. The kinetics was investigated at several initial [Ce(IV)],
[carbohydrate], [H2SO4], [S04^~], [HS04~], [surfactant], temperature, and [salt].
Figs. 3.1-3.6 show examples of some of the kinetic curves from which the rate
constants for the oxidations were obtained. As can be seen, the plots of
log(absorbance) versus time deviate from linearity: this suggests the
involvement of two reaction paths, i.e., the initial slow stage followed by a
relatively faster step."*' The time at which deviations commenced was found to
decrease with increase in [carbohydrate] and temperature whereas increasing the
acid concentration increased it. A choice was, therefore, made to study the
detailed kinetics at [H2SO4] = 1.83 mol dm"'' (except when the effect of [H2SO4]
was seen).
55
- 0 . 2 [ - ^
a> u c -0 .4 o l_
o «1 ^ S-0.6 o> o
-0.8
—
_
\ 0 ( E )
0 ( F )
1 1 1 1
X)(D)
1
—0(A)
"~-0(B)
"XD(C)
1
20 40 60 80 Time (min )
100 120
Fig. 3.1: Plots of log(absorbance) versus time showing the noncatalytic and
autocatalytic paths for the oxidation of D(+)xyIose by cerium(IV)
(= 1.0 X 10" mol dm" ) in presence of H2SO4 (= 1.83 mol dm~ ) at 40
°C. Conditions: [D(+)xylose]T = 0.0 (A), 1.0 (B), 2.0 (C), 4.0 (D), 6.0
(E) and 8.0 x 10" mol dm" (F); [CTAB] = 50.0 x lO^^mol dm~ (A).
56
-0.2
u
JD (_ O 1/)
S - 0 . 6 o
-0.8
- \
1
b(F) (
1 1
D(D)
1 .
\ ) ( C )
1
-0(A)
X)(B)
1 20 40 60 80
Time Imin )
100 120
Fig. 3.2: Plots of log(absorbance) versus time showing the noncatalytic and
autocatalytic paths for the oxidation of L(+)arabinose by cerium(I V)
(= 1.0 X IQ- mol dm-^) in presence of H2SO4 (= 1.83 mol dm~ ) at
40 °C. Conditions: [L(+)arabinose]T = 0.0 (A), I.O (B), 2.0 (C), 4,0
(D), 6.0 (E) and 8.0 x 10' mol dm" (F); [CTAB] = 50.0 x 10" mol
dm" (A).
57
Oi u c o
£i t-o w
<
-0.2
- 0 . 4 -
: i - 0 . 6 -C71 O
-0.8
60 80
T i m e (m i n )
Fig. 3.3: Plots of log(absorbance) versus time showing the noncatalytic and
autocatalytic paths for the oxidation of D(+)glucose by cerium(IV)
(= 1.0 X 10" mol dm'^) in presence of H2SO4 (= 1.83 mol dm" ) at 40
°C. Conditions: [D(+)glucose]T = 0.0 (A), 1.0 (B), 2.0 (C), 4.0 (D),
6.0 (E) and 8.0 x 10" mol dm" (F); [CTAB] = 50.0 x lO"' mol dm"
(A).
58
u c o
JD l_ o «/)
< C7» O
-0.2k
-O.^h
~ -0.6 h
-0.8 h
^ Q
^
1 1 1 1
D(D)
0(E)
1
-0 (A)
-0 (B)
X»(C)
1 20 40 60 80
Time ( m i n )
100 120
Fig. 3.4: Plots of log(absorbance) versus time showing the noncatalytic and
autocatalytic paths for the oxidation of D(+)mannose by cerium(IV)
(= 1.0 X 10" mol dm" ) in presence of H2SO4 (= 1.83 mol dm" ) at
40 °C. Conditions: [D(+)mannose]T = O.G (A), 1.0 (B), 2.0 (C), 4.0
(D), 6.0 (E) and 8.0 x 10" mol dm" (F); [CTAB] = 50.0 x lO"* mol
dm" (A).
0)
u c o L. o (/I
<
-0.2
-0.4 -
^ -0.6 -C7> O
-0.8
60 80
Ti me ( min )
Fig. 3.5: Plots of log(absorbance) versus time showing the noncatalytic and
autocatalytic paths for the oxidation of D(-)fructose by cerium(IV)
(= 1.0 X 10~ mol dm~ ) in presence of H2SO4 (= 1.83 mol dm" ) at 40
°C. Conditions: [D(-)fructose]T = CO (A), 1.0 (B), 2.0 (C), 4.0 (D),
6.0 (E) and 8.0 x 10"" mol dm~' (F); [CTAB] = 50.0 x 10^ mol dm
(A).
60
0 20 40 60 80 T i m e (m i n )
TOO 120
Fig. 3.6: Plots of log(absorbance) versus time showing tiie noncataiytic and
autocatalytic paths for the oxidation of L(-)sorbose by cerium(IV)
(= 1.0 X 10" mol dm" ) in presence of H2SO4 (= 1.83 mol dm~') at 40
°C. Conditions: [L(-)sorbose]T = 0.0 (A), 1.0 (B), 2.0 (C), 4.0 (D), 6.0
(E) and 8.0 x lO' mol dm-^(F); [CTAB] - 50.0 x 10"* mol dm~ (A).
61
The values of pseudo-first-order rate constants were evaluated from the
slopes of the initial parts (first stage) of the linear plots. An interesting feature of
this reaction is the autocatalysis, due to the catalytic role of one of the oxidation
products.
Effect of [oxidant] on the rate
The effect of concentration of oxidant on the rate constants was
determined by carrying out kinetic runs at different [Ce(IV)]. The reductant
concentrations were kept constant with 1.83 mol dm H2SO4 at 40 °C. The
pseudo-first-order rate constants (kobs) obtained at different cerium(IV)
concentrations are given in Tables 3.1-3.6. The results show that the values of
rate constants are independent of the initial oxidant concentration. Thus, it is
concluded that the order of the reaction with respect to [Ce(IV)] is one. The rate
law would then be
rate = -d[Ce(IV)]/dt = iCobs[Ce(IV)]T (3.1)
Effect of [reductant] on the rate
The effect of the concentrations of the reductants were studied keeping all
other experimental conditions constant. The results are given in Tables 3.7-
3.12.
The plots of rate constant versus [reductant] (Figs. 3.7-3.12) were found
to be linear and passing through the origin indicating first-order dependence on
[reductant].
62
TABLE 3.1:
Effect of [Ce(IV)]T on the pseudo-first-order rate constants (kobs or kvp) for the
oxidation of D(+)xylose by cerium(lV) in the absence and presence of CTAB.
Conditions: [D(+)xylose]T = 4.0 x 10" mol dm"
[H2S04]T =1.83 mol dm
Temperature = 40 °C
-3
10'[Ce(IV)]T lOXhs/k^-Cs-')
(mol dm" ) Aqueous CTAB'
"06 L9 10
0.8 1.9 2.9
0.9 1.8 2.8
1.0 1.7 2.5
1.1 1.4 2.6
1.2 1.3 2.5
1.3 1.3 2.3
' [CTAB]T = 50.0 X 10"* mol dm"
63
TABLE 3.2:
Effect of [Ce(IV)]T on the pseudo-first-order rate constants (kobs or k -) for the
oxidation of L(+)arabinose by cerium(IV) in the absence and presence of CTAB.
Conditions: [L(+)arabinose]T = 4.0 x 10 ^ mol dm ^
[H2S04]T =1.83 mol dm"
Temperature = 40 °C
10'[Ce(lV)]T 10'kobs/k>,(s-')
(mol dm" ) Aqueous CTAB' _ _ _ _ _ _
0.8 2.2 3.5
0.9 2.3 3.7
1.0 2.2 3.6
1.1 2.2 3.6
1.2 2.2 3.7
1.3 2.1 3.6
' [CTAB]T = 50.0 X 10"* mol dm"
64
TABLE 3.3:
Effect of [Ce(IV)]T on the pseudo-first-order rate constants (kobs or k^>) for the
oxidation of D(+)glucose by cerium(IV) in the absence and presence of CTAB.
Conditions: [D(+)glucose]T = 4.0 x 10" mol dm"''
[H2S04]T =1.83 mol dm
Temperature = 40 °C
-3
10'[Ce(IV)]T 10'kobs/k^(s"') -3 (mol dm"0 Aqueous CTAB'
"0 6 13 16
0.8 1.1 2.4
0.9 1.1 1.8
1.0 1.1 1.7
1.1 1.0 1.7
1.2 0.8 1.1 '
1.3 0.8 1.6
' [CTABJT = 50.0 X 10"* mol dm"
65
TABLE 3.4:
Effect of [Ce(IV)]T on the pseudo-first-order rate constants (kobs or k^) for the
oxidation of D(+)mannose by cerium(IV) in the absence and presence of CTAB.
Conditions: [D(+)mannose]T = 4.0 x 10 ^ mol dm
[H2S04]T =1.83 mol dm"
Temperature = 40 °C
10'[Ce(IV)]T To^lWMsT
(mol dm"') Aqueous CTAB'
"06 r6 2?7
0.8 1.6 2.6
0.9 1.5 2.4
1.0 1.5 2.4
1.1 1.5 2.3
1.2 1.6 4.2
1.3 1.5 2.4
' [CTAB]T = 50.0 X 10" mol dm"
66
TABLE 3.5:
Effect of [Ce(IV)]T on the pseudo-first-order rate constants (kobs or k^) for the
oxidation of D(-)fructose by cerium(IV) in the absence and presence of CTAB.
Conditions: [D(-)fructose]T = 4.0 x 10 ^ mol dm ^
[H2S04]T =1.83 mol dm~
Temperature = 40 °C
10'[Ce(IV)]T lO^kobs/k^^Cs-)
(mol dm ) Aqueous CTAB^
"06 48 5A
0.8 4.5 5.2
0.9 4.2 5.2
1.0 3.8 5.2
1.1 4.2 5.2
1.2 3.6 5.2
1.3 4.0 5.2
'[CTABJT = 50.0 X 10"* mol dm"
67
TABLE 3.6:
Effect of [Ce(IV)]T on the pseudo-first-order rate constants (kobs or k^) for the
oxidation of L(-)sorbose by cerium(IV) in the absence and presence of CTAB.
Conditions: [L(-)sorbose]T = 4.0 x 10" mol dm~
[H2S04]T =1.83 mol dm"
Temperature = 40 °C
10'[Ce(IV)]T 10^kobs/kvp(s-')
(mol dm ) Aqueous CTAB^
"06 12 5 0
0.8 3.3 4.9
0.9 3.1 4.9
1.0 3.2 4.7
1.1 3.0 4.4
1.2 2.7 4.2
1.3 2.4 4.1
'[CTABJT = 50.0 X 10"* mol dm"
68
TABLE 3.7:
Effect of [D(+)xylose]T on the pseudo-first-order rate constants (kobs or kvp) for
the oxidation of D(+)xylose by cerium(IV) in the absence and presence of
CTAB.
Conditions: [Ce(IV)]T
[H2S04]T
Temperature
= l.OxlO'^moldm"^
= 1.83moldm~^
= 40°C
10'[D(+)xylose]T
(mol dm"'')
10'kobs/kT(s"')
Aqueous CTAB'
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
0.4
0.7
1.2
1.7
1.9
2.4
2.8
3.2
3.5
0.7
1.3
2.2
2.5
3.0
3.7
4.3
5.4
5.8
' [CTAB]T = 50.0 X 10^ mol dm ,-3
69
TABLE 3.8:
Effect of [L(+)arabinose]T on the pseudo-first-order rate constants (kobs or kvp) for
the oxidation of L(+)arabinose by cerium(IV) in the absence and presence of
CTAB.
Conditions: [Ce(IV)]T
[H2S04]T
Temperature
= 1.0xlO'^moldm~^
= 1.83moldm"^
= 40°C
10lL(+)arabinose]i
(mol dm~ )
lO'kobs/kH'Cs"')
Aqueous CTAB'
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
0.8
1.2
1.5
2.2
2.6
3.5
4.2
5.4
5.9
1.0
1.5
2.8
3.6
4.2
5.2
6.1
6.5
7.5
,-A '[CTABJT = 50.0 X 10^ mol dm
70
TABLE 3.9:
Effect of [D(+)glucose]T on the pseudo-first-order rate constants (kobs or k^v) for
the oxidation of D(+)glucose by cerium(IV) in the absence and presence of
CTAB.
Conditions: [Ce(IV)]T
[H2S04]T
Temperature
= 1.0x lO'^moldm"^
= 1.83moldm"^
= 40°C
10^[D(+)glucose]i
(mol dm~ )
lO^kobs/kTCs"')
Aqueous CTAB'
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
0.3
0.4
0.8
1.1
1.3
1.8
1.9
2.3
2.5
0.4
1.0
1.2
1.7
1.8
2.3
2.6
3.0
3.3
-A 1CTAB]T = 5 0 . 0 X 1 0 ^ mol dm
71
TABLE 3.10:
Effect of [D(+)mannose]T on the pseudo-first-order rate constants (kobs or kvp) for
the oxidation of D(+)mannose by cerium(IV) in the absence and presence of
CTAB.
Conditions: [Ce(IV)]T
[H2S04]T
Temperature
= 1.0xlO"^moldm"^
= 1.83moldm"^
= 40°C
lO^kobs/k^-Cs"') 10^[D(+)mannose]i
(mol dm"^) Aqueous CTAB'
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
0.4
0.7
1.4
1.5
1.7
2.3
2.5
3.1
3.3
0.6
1.0
1.6
2.4
3.1
3.5
3.7
4.3
5.0
-4 1 C T A B ] T = 50.0 X 10-^ mol dm
72
TABLE 3.11:
Effect of [D(-)fructose]T on the pseudo-first-order rate constants (kobs or 4/) for
the oxidation of D(-)fructose by cerium(IV) in the absence and presence of
CTAB.
Conditions: [Ce(IV)]T
[ H 2 S 0 4 ] T
Temperature
= 1.0xlO"^moldm"^
= 1.83 mol dm
= 40°C
-3
10'kobs/kT(s-') 10lD(-)fructose]n
(mol dm~ ) Aqueous CTAB'
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
1.0
1.9
2.6
3.8
5.1
6.1
6.6
8.9
10.0
1.2
2.4
3.7
5.2
6.1
7.8
9.0
10.5
11.9
' [CTAB]T = 50.0 X 10"* mol dm"
73
TABLE 3.12:
Effect of [L(-)sorbose]T on the pseudo-first-order rate constants (kobs or k /) for
the oxidation of L(-)sorbose by cerium(IV) in the absence and presence of
CTAB.
Conditions: [Ce(IV)]T
[H2S04]T
Temperature
= 1.0x 10"^moldm"^
= 1.83moldm~^
= 40°C
lO^kobs/ks'Cs-') 101L(-)sorbose]T
(mol dm"'') Aqueous CTAB'
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
0.8
1.6
2.5
3.2
3.8
4.2
5.2
5.6
6.9
1.4
2.7
4.0
4.7
6.0
7.1
8.2
9.5
11.1
' [CTAB]T = 50.0 X 10"* mol dm~
74
6.0 -
(/)
c o w c o u (U
o (-o
0.0 2.0 4.0 6.0 8-0 102[:D( + )xy lose3(moldm-3)
Fig. 3.7: Plots of rate constants versus [D(+)xylose]T for the oxidation of
D(+)xylose by cerium(IV). Conditions: [Ce(IV)]T = 1.0 x 10" mol
dm-\ [H2S04JT = 1.83 mol dm'^ [CTAB]T = 0.0 (O), 50.0 x lO"* mol
dm" ( • ) , temperature = 40 °C.
75
^H. 5.0 -c o
c o
o
2 "X
10 C L(+)Qrabinose3lniol dm'"-')
Fig. 3.8: Plots of rate constants versus [L(+)arabinose]T for the oxidation of
L(+)arabinose by cerium(IV). Conditions: [Ce(IV)]T = 1.0 x 10" mol
&m\ [H2S04]T = 1.83 mol dm^^ [CTABJT = 0.0 (O), 50.0 x 10" mol
dm" ( • ) , temperature = 40 °C.
76
- • -c o w c o (J
o
0.0 2.0 4.0 6.0 8.0
10^[ ID(+ )glucose3{nnoldm-3 )
Fig. 3.9: Plots of rate constants versus [D(+)glucose]T for the oxidation of
D(+)gIucose by cerium(IV). Conditions: [Ce(IV)]T = 1.0 x 10" mol -1
dm"^ [H2S04]T = 1.83 mol dm'', [CTAB]T = 0.0 (O), 50.0 x 10^ mol
dm" ( • ) , temperature = 40 °C.
77
0.0 2.0 ^.0 6.0 8.0 lO^CD{ + )manno5eI](mol dm- 3)
Fig. 3.10: Plots of rate constants versus [D(+)mannose]T for the oxidation of
D(+)mannose by cerium(IV). Conditions: [Ce(IV)]x = 1.0 x 10" mol
dm"\ [H2S04]T = 1.83 mol dm"^ [CTAB]T = 0.0 (O), 50.0 x 10^
mol dm" ( • ) , temperature = 40 °C.
78
2.0 4.0 6.0 8.0 10^[ :D( - ) f ruc toseI ] (moldm-3)
Fig. 3.11: Plots of rate constants versus [D(-)fructose]T for the oxidation of
D(-)fructose by cerium(IV). Conditions: [Ce(IV)]T = 1.0 x 10"' mol -3 dm'^ [H2S04]T = 1.83 mol drn'^ [CTABJx = 0.0 (O), 50.0 x 10
mol dm" ( • ) , temperature = 40 °C.
-4
79
0,0 2.0 A,0 6.0 8.0
1 0 2 C L ( - ) so rbose I ] (mo ldm-3)
Fig. 3.12: Plots of rate constants versus [L(-)sorbose]T for the oxidation of
L(-)sorbose by cerium(IV). Conditions: [Ce(IV)]T = 1-0 x 10" mol
dm~\ [H2S04]T = 1.83 mol dm~^ [CTAB]T = 0.0 (O), 50.0 x 10^
mol dm~ ( • ) , temperature = 40 °C.
80
Effect of [H2SO4] on the rate
In order to see the role of H2SO4, kinetic runs were performed at different
[H2SO4] (assuming [H2SO4] = [H ]) in the range (0.73-3.67 mol dm~ ). At fixed
[reductant] (= 4.0 x 10" mol dm" ), [Ce(IV)] (= 1.0 x 10" mol dm'^), and
temperature (= 40 °C), the rate constants were found to decrease with increase in
[H2SO4] (Tables 3.13-3.18). Due to involvement of large number of proton
dependent equilibria in the cerium(IV),^'° the exact computation of [H""] and
interpretation of [H" ] dependence are not possible. However, the inhibition of
reaction rate by the addition of H2SO4 may be explained as due to the removal of
reactive species of cerium(IV).
Effect of [S04 "] on the rate
It is well established that cerium(IV) forms a variety of complexes with
S04^". Therefore, kinetic experiments were carried out in presence of varying
amounts of S04^~. It was observed that the rate constant increased with
increasing [SO4 ~], indicating involvement of Ce(IV)-sulfato species as the
reactive species. The results at different [S04^~] at fixed [oxidant], [reductant],
[H2SO4], and temperature are summarized in Tables 3.19-3.24.
Effect of [HS04~] on the rate
Tables 3.19-3.24 also summarize the effect of [HS04~] on the rates of
reactions. The reaction rates are retarded by increase in [HS04~]. The retardation
81
TABLE 3.13:
Effect of [H2S04]T on the pseudo-first-order rate constants (kobs or k^>) for the
oxidation of D(+)xylose by cerium(IV) in the absence and presence of CTAB.
Conditions:
[H2S04]T
(mol dm"'')
0.73
1.10
1.47
1.83
2.20
2.57
2.94
3.30
3.67.
[Ce(IV)]T
[D(+)xylose]T
Temperature
= 1.0x10"
= 4.0x10"
= 40°C
10'kobs
^ mol dm
^ mol dm~
/k4-(s-')
Aqueous
2.0
2.1
1.8
1.7
1.3
1.1
1.1
0.9
0.8
CTAB'
turbidity
turbidity
turbidity
2.5
2.5
2.4
2.3
2.1
2.0
-A TCTABJT =50.0x10^ mol dm
82
TABLE 3.14:
Effect of [H2S04]T on the pseudo-first-order rate constants (kobs or k4>) for the
oxidation of L(+)arabinose by cerium(IV) in the absence and presence of CTAB.
Conditions: [Ce(IV)]T
[L(+)arabinose]T
Temperature
= 1.0xlO"^moldm"^
= 4.0xl0"^moldm"^
= 40°C
[H2S04]T
(mol dm~ )
0.73
1.10
1.47
1.83
2.20
2.57
2.94
3.30
3.67
lO' kobs/kTCs"')
Aqueous
3.5
3.1
2.9
2.2
1.9
1.7
1.7
1.3
1.1
CTAB'
turbidity
turbidity
turbidity
3.6
3.0
2.5
2.2
2.0
1.7
'[CTABJT = 50.0 X 10"* mol dm'
83
TABLE 3.15:
Effect of [H2S04]T on the pseudo-first-order rate constants (kobs or k /) for the
oxidation of D(+)glucose by cerium(IV) in the absence and presence of CTAB.
Conditions:
[H2S04]T
(mol dm~ )
0.73
1.10
1.47
1.83
2.20
2.57
2.94
3.30
3.67
[Ce(IV)]T
[D(+)glucose]T
Temperature
= 1.0x10"
= 4.0x10"
= 40°C
10'kobs
^ mol dm
^ mol dm""'
/k4.(s"')
Aqueous
1.0
1.2
0.9
1.1
0.9
0.8
0.7
0.6
0.5
CTAB'
turbidity
turbidity
turbidity
1.7
1.7
1.5
1.7
1.5
1.7
' [CTAB]T = 50.0 X 10" mol dm"
84
TABLE 3.16:
Effect of [H2S04]T on the pseudo-first-order rate constants (kobs or kvj-) for the
oxidation of D(+)mannose by cerium(IV) in the absence and presence of CTAB.
Conditions:
[H2S04]T
(mol dm"' )
0.73
1.10
1.47
1.83
2.20
2.57
2.94
3.30
3.67
[Ce(IV)]T
[D(+)mannose]T
Temperature
= 1.0xl0~^moldm"^
= 4.0xl0"^moldm~^
= 40°C
10'kobs/k>,(s-')
Aqueous
1.5
1.5
1.5
1.5
1.2
1.0
0.8
0.7
0.5
CTAB'
turbidity
turbidity
turbidity
2.4
2.1
1.8
1.7
1.2
1.2
'[CTAB]T = 50.0 X 10^ mol dm
85
TABLE 3.17:
Effect of [H2S04]T on the pseudo-first-order rate constants (kobs or k>{-) for the
oxidation of D(-)fructose by cerium(IV) in the absence and presence of CTAB.
Conditions: [Ce(IV)]T
[D(-)fructose]T
Temperature
= 1.0xlO"^moldm"^
= 4.0xl0~^moldm~^
= 40°C
[H2S04]T
(mol dm~ )
0.73
1.10
1.47
1.83
2.20
2.57
2.94
3.30
3.67
10'*kobs/k>p(s-')
Aqueous
-
6.1
5.7
3.8
3.7
3.2
2.5
2.2
2.0
CTAB'
turbidity
turbidity
turbidity
5.2
4.5
3.5
2.9
2.5
2.3
-4 1CTAB]T = 50.0 X 10^ mol dm
86
TABLE 3.18:
Effect of [H2S04]T on the pseudo-first-order rate constants (kobs or k^) for the
oxidation of L(-)sorbose by cerium(IV) in the absence and presence of CTAB.
Conditions: [Ce(IV)]T
[L(-)sorbose]T
Temperature
= 1.0xlO"^moldm"^
= 4.0x 10"^moldm~^
= 40°C
[ H 2 S 0 4 ] T
(mol dm"'')
0.73
1.10
1.47
1.83
2.20
2.57
2.94
3.30
3.67
10'kobs/k4.(s-')
Aqueous
4.2
4.6
3.5
3.2
2.9
2.4
2.1
1.9
1.5
CTAB'
turbidity
turbidity
turbidity
4.7
4.1
3.6
3.2
2.7
2.6
-4 1CTAB]T = 50.0 X 10" mol dm
87
TABLE 3.19:
Effect of [Na2S04]T and [NaHS04]T on the pseudo-first-order rate constants
(kobs) for the oxidation of D(+)xylose by cerium(IV) in aqueous medium.
Conditions: [Ce(IV)]T = 1.0 x 10" mol dm~
[D(+)xylose]T = 4.0 x 10" mol dm~
[H2S04]T =1.83 mol dm~
Temperature = 40 °C
10'[Na2SO4]T lOXbs 10'[NaHSO4]T lOXbs
(moldm-^) (s~ ) (mol dm~ ) (s~')
0.0
1.0
2.0
4.0
5.0
10.0
1.7
1.7
1.8
1.8
1.9
2.1
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
1.7
1.7
1.6
1.5
1.3
1.2
1.1
0.9
0.9
88
TABLE 3.20:
Effect of [Na2S04]T and [NaHS04]T on the pseudo-first-order rate constants
(kobs) for the oxidation of L(+)arabinose by cerium(IV) in aqueous medium.
Conditions: [Ce(IV)]T = 1.0 x 10" mol dm"
[L(+)arabinose]T =4.0x10"^ mol dm"
[H2S04]T =1.83 mol dm~
Temperature = 40 °C
10'[Na2SO4]T
(mol dm~ )
0.0
1.0
2.0
4.0
5.0
10.0
lOXbs
(s-')
2.2
2.3
2.4
2.9
3.3
4.1
10'[NaHSO4]T
(mol dm"'')
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
lOXbs
(s-')
2.2
2.0
1.9
1.8
1.6
1.5
1.4
1.2
1.0
89
TABLE 3.21:
Effect of [Na2S04]T and [NaHS04]T on the pseudo-first-order rate constants
(kobs) for the oxidation of D(+)glucose by cerium(IV) in aqueous medium.
Conditions: [Ce(IV)]T
[D(+)glucose]T
[H2S04]T
Temperature
10'[Na2SO4]T lOXbs
(moldm-^) (s"')
0.0 1.1
1.0 1.2
2.0 1.5
4.0 1.6
5.0 1.7
= 1.0
= 4.0
x l O '
x l O '
^ mol dm
^ mol dm~
= 1.83moldm'^
= 40' =C
10'[NaHSO4]T
(mol dm"^)
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
lOXbs
(s-')
1.1
1.1
1.0
0.9
0.8
0.8
0.7
0.7
0.6
90
TABLE 3.22:
Effect of [Na2S04]T and [NaHS04]T on the pseudo-first-order rate constants
(kobs) for the oxidation of D(+)mannose by cerium(IV) in aqueous medium.
Conditions: [Ce(IV)]T = 1.0 x 10" mol dm"
[D(+)mannose]T = 4.0 x 10" mol dm~
[H2S04]T =1.83 mol dm~
Temperature = 40 °C
10'[Na2SO4]T
(mol dm~ )
0.0
1.0
2.0
4.0
5.0
10.0
lOXbs
(s-')
1.5
1.5
1.6
1.7
1.7
1.8
10'[NaHSO4]T
(mol dm" )
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
lOXbs
(s-')
1.5
1.5
1.4
1.3
1.3
1.2
1.0
0.9
0.8
91
TABLE 3.23:
Effect of [Na2S04]T and [NaHS04]T on the pseudo-first-order rate constants
(kobs) for the oxidation of D(-)fructose by cerium(IV) in aqueous medium.
Conditions: [Ce(IV)]T = 1.0 x 10" mol dm~
[D(-)fructose]T =4.0x10~^ mol dm"
[H2S04]T =1.83 mol dm"
Temperature = 40 °C
WfNa^SOjT lOXbs lO [NaHS04]T lOXbs
(moldm"^) (s"') (mol dm" ) (s"')
0.0
1.0
2.0
4.0
5.0
3.8
3.8
4.0
4.1
4.2
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
3.8
3.5
3.1
2.8
2.3
2.1
1.8
1.5
1.1
92
TABLE 3.24:
Effect of [Na2S04]T and [NaHS04]T on the pseudo-first-order rate constants
(kobs) for the oxidation of L(-)sorbose by cerium(IV) in aqueous medium.
Conditions: [Ce(IV)]T = 1.0 x 10" mol dm"
[L(-)sorbose]T =4.0x10"^ mol dm"
[H2S04]T =1.83 mol dm"
Temperature = 40 °C
10'[Na2SO4]T lOXbs 10'[NaHSO4]T lOXbs
(moldm"^) (s"') (mol dm" ) (s"')
0.0
1.0
2.0
4.0
5.0
0.0
3.2
3.2
3.3
3.4
3.5
3.7
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
3.2
3.2
2.9
2.8
2.6
2.2
2.2
2.0
1.9
93
in the rates with increasing [1 804""] suggests the removal of the reactive species
of cerium(IV).
Effect of temperature on the rate
The reactions were performed at different temperatures (30-50 °C).
Tables 3.25-3.30 record the values of pseudo-first-order rate constants obtained
at the respective temperatures. The Arrhenius plots of log k {i.e., kobs or k^)
versus 1/T were linear and activation energies (Ea) were evaluated from the
slopes of such plots for the oxidation of carbohydrates by cerium(IV). The
enthalpies and entropies of activation (AH"* and AS**) were calculated using
Eyring equation
k = (keT/h) exp (AS /R) exp (-AH^/RT) (3.2)
where the symbols have their usual meaning. Tables 3.25-3.30 also record the
thermodynamic parameters.
Effect of [surfactant] on the rate
Several redox reactions in the micellar media are influenced by the
hydrophobic and electrostatic forces and, for a given set of reactions, the
observed rate depends on the extent of association between the reactants and
micellar aggregates. Therefore, in order to see the role of surfactants, a series of
kinetic runs were performed in presence of varying amounts of sodium dodecyl
94
TABLE 3.25:
Effect of temperature on the pseudo-first-order rate constants (kobs or k4 ) for the
oxidation of D(+)xylose by cerium(IV) in the absence and presence of CTAB.
Conditions: [Ce(IV)]T
[D(+)xylose]T
[H2S04]T
Temperature
(°C)
30
35
40
45
50
Parameters
Ea (kJmoI"')
AH (kJmoi"')
AS*(JK-'mor')
= 1.0x10"^
= 4.0x10"^
' mol dm ^
' mol dm~
= 1.83moldm"^
lO'kob: ,/k4-(s-')
Aqueous
0.5
0.9
1.7
3.0
5.6
99
96
-10
CTAB'
0.8
1.4
2.5
4.1
7.5
93
90
-27
-4 1CTAB]T = 50.0 X 10" mol dm
95
TABLE 3.26:
Effect of temperature on the pseudo-first-order rate constants (kobs or kvp) for the
oxidation of L(+)arabinose by cerium(IV) in the absence and presence of CTAB.
Conditions: [Ce(IV)]T = 1.0 x 10" mol dm"
[L(+)arabinose]T = 4.0 x 10" mol dm""'
[H2S04]T =1.83 mol dm"
Temperature
(°C)
30
35
40
45
50
Parameters
Ea (kJmof')
AH* (kJmoP')
AS^'CJK-'mor')
lO'kobs/k^-Cs-')
Aqueous
0.8
1.5
2.2
5.2
6.8
98
96
-17
CTAB'
1.0
1.8
3.6
6.0
8.6
89
86
-35
'[CTABJT = 50.0 X 10"* mol dm'
96
TABLE 3.27:
Effect of temperature on the pseudo-first-order rate constants (kobs or k^) for the
oxidation of D(+)glucose by cerium(IV) in the absence and presence of CTAB.
Conditions: [Ce(IV)]T
[D(+)gIucose]T
[H2S04]T
Temperature
(°C)
30
35
40
45
50
Parameters
Ea (kJmol"')
AH'' (kJmor')
AS* (JK"'mor')
= 1.0x10"-
= 4.0x10"^
' mol dm ^
• mol dm~^
= 1.83moldm~^
10'kobs/kvp(s~')
Aqueous
0.3
0.9
1.1
2.0
3.3
92
89
-54
CTAB'
0.6
1.2
1.7
2.7
4.3
88
85
-84
-4 ' [CTAB]T =50.0xlO-^moldm -3
97
TABLE 3.28:
Effect of temperature on the pseudo-first-order rate constants (kobs or ^^v) for the
oxidation of D(+)mannose by cerium(IV) in the absence and presence of CTAB.
Conditions: [Ce(IV)]T
[D(+)mannose]T
[H2S04]T
= 1.0xlO"^moldm"^
= 4.0xl0~^moldm"^
= 1.83moldm"^
Temperature
(°C)
30
35
40
45
50
Parameters
Ea(kJmor')
AH* (kJmof')
AS'' (JK"'mol'')
10'*kobs/k>,(s-')
Aqueous
0.4
0.8
1.5
2.4
3.8
94
91
-28
CTAB'
0.7
1.4
2.4
3.3
4.8
79
76
-73
-4 1CTAB]T = 50.0 X 10" mol dm
98
TABLE 3.29:
Effect of temperature on the pseudo-first-order rate constants (kobs or k^) for the
oxidation of D(-)fructose by cerium(IV) in the absence and presence of CTAB.
Conditions: [Ce(IV)]T
[D(-)fructose]T
[H2S04]T
Temperature
(°C)
30
35
40
45
50
Parameters
Ea (kJmor')
AH* (kJmor')
AS* (JK"'mor')
= 1.0xlO"^moldm'^
= 4.0x 10~^moldm'
= 1.83moldm"^
lO'^kobs/kH'Cs-')
Aqueous
1.1
2.5
3.8
8.0
12.2
99
96
-17
CTAB'
1.7
3.0
5.2
9.0
14.1
87
85
-38
' [CTAB]T = 50.0 X 10" mol dm"
99
TABLE 3.30:
Effect of temperature on the pseudo-first-order rate constants (kobs or k<v) for the
oxidation of L(-)sorbose by cerium(IV) in the absence and presence of CTAB.
Conditions: [Ce(IV)]T
[L(-)sorbose]T
[H2S04]T
= 1.0xlO"^moldm"^
= 4.0x 10"^moldm~^
= 1.83moldm"^
Temperature
CO 30
35
40
45
50
Parameters
Ea (kJmor')
AH* (kJmor')
AS'' (JK"'mor')
lO'kobs/k^-Cs-')
Aqueous
1.0
1.8
3.2
5.1
9.0
90
87
-34
CTAB'
1.7
2.5
4.8
7.7
13.2
86
83
-43
TCTABJT = 50.0 X 10^ mol dm -3
100
sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) at constant
[Ce(IV)] (= 1.0 X 10" mol dm" ), [carbohydrate] (= 4.0 x 10" mol dm" ),
[H2SO4] (= 1.83 mol dm'^), and temperature (= 40 °C). The rate constants are
summarized in Tables 3.31 - 3.36. It is observed that the rate constants increase
with increase in [CTAB] but SDS has no effect (Figs. 3.13-3.18).
In order to see whether the bromide ion is capable of a reaction with
cerium(IV) under our kinetic conditions, some experiments were also performed
in the absence of carbohydrates (cerium(IV) + CTAB + H2SO4). The absorbance
of cerium(IV) remained constant for the entire range of [CTAB] used in the
kinetic experiments (Figs. 3.1-3.6, A). This suggests that the oxidation of
bromide ion by cerium(IV) is not involved even as a side reaction in the
carbohydrate oxidation by cerium(IV) in presence of CTAB. Furthermore, a
series of kinetic experiments performed with varying [Br~] ((10.0 to 50.0) x 10~
mol dm" ) at constant [H2SO4] = 1.83 mol dnf\ [Ce(IV)] = 1.0 x 10" mol dnf\
and temperature = 40 °C showed constancy of absorbance upto 90 min (typical
time required for complete oxidation of carbohydrates under our kinetic
conditions). These results confirm that there is no reduction of cerium(IV) by
bromide ion.
Effect of [oxidant], [reductant], [H2SO4I and temperature on the rate in
micellar medium
To see the effects of [oxidant], [reductant], [H2SO4] and temperature, and
to further confirm the mechanism {vide infra), a. series of kinetic experiments
101
TABLE 3.31:
Effect of [surfactant]! on the pseudo-first-order rate constants (k -) for the
oxidation of D(+)xylose by cerium(IV).
Conditions: [Ce(IV)]T
[D(+)xylose]T
[H2S04]T
Temperature
10'[surfactant]T 10' k^. (s"')
(mol dm" )
0.0
10.0
20.0
30.0
40.0
50.0
75.0
100.0
125.0
150.0
CTAB
1.7
1.9
2.1
2.2
2.5
2.5
2.8
2.8
3.0
3.1
= 1.0
= 4.0
xlO"^moidm"^
xlO"^moldm"^
= 1.83 mol dm'
= 40' C
SDS
1.7
1.6
1.6
1.6
1.7
1.7
1.6
1.6
1.7
1.6
10' Weal (S-')
CTAB
-
1.9
2.1
2.2
2.4
2.5
2.7
2.9
3.0
3.1
102
TABLE 3.32:
Effect of [surfactant]! on the pseudo-first-order rate constants (k^) for the
oxidation of L(+)arabinose by cerium(IV).
Conditions: [Ce(IV)]T
[L(+)arabinose]T
[H2S04]T
Temperature
10'[surfactant]T 10'kvp(s"')
(mol dm~ )
0.0
10.0
20.0
30.0
40.0
50.0
75.0
100.0
125.0
150.0
CTAB
2.2
2.3
2.9
3.1
3.3
3.6
3.7
3.7
4.0
4.2
= 1.0
= 4.0
X 10"' mol dm"'
X lO'^moldm"^
= 1.83 mol dm"
= 40 °C
SDS
2.2
-
-
-
-
2.1
2.3
2.1
-
2.2
10^k>peal(s-')
CTAB
-
2.6
2.9
3.1
3.3
3.5
3.7
3.9
4.1
4.2
oxidation of D(+)glucose by cerium(IV).
Conditions: [Ce(IV)]T
[D(+)glucose]T
[H2S04]T
Temperature
= 1.0x 10" molclm~^
= 4.0x 10~^moldm"^
= 1.83moldm"^
= 40°C
10''[surfactant]T
(mol dm~ )
0.0
10.0
20.0
30.0
40.0
50.0
75.0
100.0
125.0
150.0
10'k4>(s~')
CTAB
1.1
1.2
1.3
1.4
1.4
1.7
1.7
1.8
1.9
2.0
SDS
1.1
-
1.1
1.2
-
1.1
-
1.1
-
1.0
10'k^cal(s-')
CTAB
-
1.2
1.3
1.4
1.4
1.5
1.7
1.8
1.9
2.0
104
TABLE 3.34:
Effect of [surfactantjj on the pseudo-first-order rate constants (k^) for the
oxidation of D(+)mannose by cerium(IV).
Conditions:
10''[surfacta
(mol dm~ )
0.0
10.0
20.0
30.0
40.0
50.0
75.0
100.0
125.0
150.0
[Ce(IV)]T
[D(+)mannose]T
[H2S04]T
Temperature
•nt]T lo'kH^Cs-';
CTAB
1.5
1.7
1.8
2.1
2.2
2.4
2.6
2.7
2.8
2.9
= 1.0
= 4.0
X 10"^moldm"^
X 10" mol dm"^
= 1.83 mol dm"
= 40"
)
=C
SDS
1.5
1.5
1.5
1.4
-
1.5
-
1.5
1.5
1.5
lO'kH.ealCs"')
CTAB
-
1.7
1.8
2.0
2.1
2.3
2.5
2.7
2.9
3.0
105
TABLE 3.35:
Effect of [surfactant]! on the pseudo-first-order rate constants (k^) for the
oxidation of D(-)fructose by cerium(IV).
Conditions: [Ce(IV)]T
[D(-)fructose]T
[H2S04]T
Temperature
10'*[surfactant]T lO' ks^Cs"')
(mol dm"'')
0.0
10.0
20.0
30.0
40.0
50.0
75.0
100.0
125.0
150.0
CTAB
3.8
4.4
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
= 1.0
= 4.0
X 10" mol dm"^
X 10"^moldm"^
= 1.83 mol dm"
= 40' =C
SDS
3.8
-
-
-
-
3.8
3.8
4.2
-
3.8
10'k.peal(s"')
CTAB
-
4.2
4.6
4.8
5.0
5.2
5.4
5.6
5.8
5.9
106
TABLE 3.36:
Effect of [surfactant]! on the pseudo-first-order rate constants (k *) for the
oxidation of L(-)sorbose by cerium(IV).
Conditions: [Ce(IV)]T
[L(-)sorbose]T
[H2S04]T
Temperature
= 1.0xlO~^moldm"^
= 4.0x lO'^moldm"^
= 1.83 mol dm
= 40°C
-3
10''[surfactant]T
(mol dm"" )
0.0
10.0
20.0
30.0
40.0
50.0
75.0
100.0
125.0
150.0
10'k>p(s-')
CTAB
3.2
3.6
4.0
4.2
4.4
4.7
5.2
5.5
5.7
6.0
SDS
3.2
-
-
-
-
3.2
3.2
3.1
-
3.1
lO' k PcalCs-')
CTAB
-
3.6
3.9
4.2
4.5
4.7
5.1
5.4
5.6
5.8
107
0.0 50.0 100.0
10 * C su r foc ton tH Imoldm' '^)
150.0
Fig. 3.13: Effect of [surfactantji on the rate constant for the oxidation of
D(+)xylose by cerium(IV). Conditions: [Ce(IV)]T = 1.0 x 10~" mol
dm--\ [D(+)xylose]T = 4.0 x lO ^ mol dm'^ [H2S04]T = 1.83 mol
dm"\ temperature = 40 °C.
108
4.5 -
u)
O
> 3.0 -
50.0 100,0
lO^Csurfactant J ( m o l d m - 3 )
150.0
Fig. 3.14: Effect of [surfactant]! on the rate constant for the oxidation of
L(+)arabinose by cerium(IV). Conditions: [Ce(IV)]x = 1.0 x lO'"'
mol dm"\ [L(+)arabinose]T = 4.0 x 10~ mol dm~\ [H2S04]T = 1.83
mol dm"'', temperature = 40 °C.
109
^ 1.5 -
in
>
-J
0.0 50.0 100.0
10^ C surfQctontH (moldm-3)
150-0
Fig. 3.15: Effect of [surfactantjx on the rate constant for the oxidation of
D(+)glucose by cerium(IV). Conditions: [Ce(IV)]T = 1.0 x 10' mol
dm-\ [D(+)gJucosejT = 4.0 x lO"' mol dm-^ [H2S04]T = 1.83 mol
dm"-', temperature = 40 °C.
5 0.0 100.0 10^ C su r fac t an t D ( m o l d m " ^ )
1500
Fig. 3.16: Effect of [surfactantji on the rate constant for the oxidation of
D(+)mannose by cerium(IV). Conditions: [Ce(IV)]T = 1.0 x 10" mol
dirf\ [D(+)mannose]T = 4.0 x 10" mol dm~^ [H2S04]T = 1.83 mol
dm~ , temperature = 40 °C.
] ]
1 (A
o
3.0 50.0 100.0
lO^Csurfactant D (moldm-^)
150.0
Fig. 3.17: Effect of [surfactantjT on the rate constant for the oxidation of
D(-)fructose by cerium(IV). Conditions: [Ce(IV)]T = 1.0 x 10" mol
dm-^ [D(-)fructose]T = 4.0 x lO ^ mol dm-^ [H2S04]T = 1.83 mol
dm~ , temperature = 40 °C.
12
6 . 0 -
in
o
0.0 50-0 100.0 150-0
1 0 [ I s u r f Q c t a n t I l ( m o l d m - 3 )
Fig. 3.18: Effect of [surfactantjx on the rate constant for the oxidation of
L(-)sorbose by cerium(IV). Conditions: [Ce(IV)]x = 1.0 x 10~ mol
dm~\ [L(-)sorbose]T = 4.0 x 10" mol dm'^ [H2S04]T = 1.83 mol
dm"\ temperature = 40 °C.
113
were performed at constant [CTAB]. The k^> - values, obtained as functions of
the above variables, are summarized in Tables 3.1-3.18 and 3.25-3.30. The
effect of [carbohydrate] on the kvp in the presence of CTAB are shown in Figs.
3.7-3.12. The behavior on variation of [oxidant], [reductant], [H2SO4], and
temperature was identical to the aqueous medium and the kinetics follow the
same pattern, i.e., first-order in [Ce(IV)] and [carbohydrate] and inverse order in
[H2SO4]. These observations undoubtedly establish that the mechanism of
oxidation of the carbohydrates by cerium(IV) in presence of CTAB remains the
same as in aqueous medium.
Effect of [salt] on the rate in micellar medium
The effect of added salts on the rate were also explored because salts, as
additives, in micellar systems acquire a special place due to their ability to
induce structural changes which may, in turn, modify the substrate-surfactant
interactions. The salt effect on the micelle catalyzed cerium(IV)-carbohydrate
reactions were studied in presence of CTAB micelles at 40 °C.
The observed data in the presence of inorganic salts (Na2S04, NaNOs and
NaCl) are recorded in Tables 3.37-3.42 and shown graphically in Figs. 3.19-
3.24.
114
TABLE 3.37:
Effect of [saltjj on the pseudo-first-order rate constants (k p) for the oxidation of
D(+)xylose by cerium(IV) in presence of CTAB.
Conditions: [Ce(IV)]T = 1.0 x 10" mol dm"
[D(+)xylose]T = 4.0 x 10" mol dm"
[H2S04]T =1.83moldm~^
[CTAB]T = 50.0 X lO"* mol dm"
Temperature = 40 °C
10'[salt]T 10'k4>(s-')
(mol dm" ) Na2S04 NaNOj NaCl
~aO iTs 15 Z5
1.0 2.5 2.5 2.5
2.5 2.4 2.4 2.5
5.0 2.1 2.3 2.5
7.5 2.0 2.2 2.4
10.0 1.8 2.1 2.3
12.5 1.8 2.1 2.2
15.0 1.7 2.0 2.2
20.0 1.5 1.9 2.1
115
TABLE 3.38:
Effect of [salt]T on the pseudo-first-order rate constants (kip) for the oxidation of
L(+)arabinose by cerium(IV) in presence of CTAB.
Conditions:
10'[salt]T
(mol dm~ )
0.0
1.0
2.5
5.0
7.5
10.0
12.5
15.0
20.0
[Ce(IV)]T
[L(+)arabinose]T
[H2S04]T
[CTABJT
Temperature
10'kt
NazSO,
3.6
3.5
3.3
3.1
2.8
2.7
2.6
2.4
2.2
= 1.0x10"^
^4.0x 10"
mol dm ^ "3
mol dm -• 1.83 mol dm'
= 50.0x10
= 40°C
(s-)
4
" mol dm ^
NaNOs
3.6
3.6
3.5
3.3
3.2
3.1
2.9
2.8
2.6
NaCl
3.6
3.6
3.5
3.4
3.3
3.2
3.2
3.1
2.9
116
TABLE 3.39:
Effect of [saltJT on the pseudo-first-order rate constants (kip) for the oxidation of
D(+)glucose by cerium(IV) in presence of CTAB.
Conditions: [Ce(IV)]T
[D(+)glucose]T
[H2S04]T
[CTAB]T
Temperature
= 1.0xlO"^moldm~^
= 4.0x lO'^moldm"^
= 1.83moldm"^
= 50.0xlO"^moldm'^
= 40°C
10'[salt]T
(mol dm~ )
"To 1.0
2.5
5.0
7.5
10.0
12.5
15.0
20.0
lO'kH-Cs"')
Na2S04
TT
1.7
1.5
1.5
1.4
1.2
1.2
1.0
1.0
NaNOa
TT
1.7
1.6
1.5
1.4
1.4
1.3
1.3
1.2
NaCl
1.7
1.7
1.7
1.6
1.6
1.5
1.5
1.4
1.4
117
TABLE 3.40:
Effect of [saltJT on the pseudo-first-order rate constants (k<i>) for the oxidation of
D(+)mannose by cerium(IV) in presence of CTAB.
Conditions: [Ce(IV)]T
[D(+)mannose]T
[H2S04]T
[CTABJT
Temperature
= 1.0xlO"^moldm~^
-4 .0x 10"^moldm"^
= 1.83moldm~^
= 50.0xlO"'moldm"^
= 40°C
irfsaith (mol dm~ )
~~ao 1.0
2.5
5.0
7.5
10.0
12.5
15.0
20.0
10'kvp(s-')
Na2S04
2.2
2.1
1.9
1.7
1.5
1.5
1.4
1.3
NaN03
"Z4
2.2
2.1
2.1
2.0
1.8
1.8
1.7
1.6
NaCl
2.4
2.4
2.4
2.4
2.2
2.2
2.1
2.1
1.9
118
TABLE 3.41:
Effect of [salt]! on the pseudo-first-order rate constants (kyy) for the oxidation of
D(-)fructose by cerium(IV) in presence of CTAB.
Conditions: [Ce(IV)]T
[D(-)fructose]T
[H2S04]T
[CTAB]T
Temperature
= 1.0xlO~^moldm"^
= 4.0xl0~^moldm~^
= 1.83moldm~^
= 50.0xlO"*moldm"^
= 40°C
ir[salt]T
(mol dm"'')
"To 1.0
2.5
5.0
7.5
10.0
12.5
15.0
20.0
Na2S04
T2 5.1
5.0
4.7
4.2
3.8
3.5
3.2
2.9
NaN03
~52
5.2
5.1
4.9
4.7
4.6
4.3
4.1
3.8
NaCl
5.2
5.2
5.1
5.0
4.8
4.8
4.5
4.3
4.1
119
TABLE 3.42:
Effect of [salt]T on the pseudo-first-order rate constants (k^-) for the oxidation of
L(-)sorbose by cerium(IV) in presence of CTAB.
Conditions:
10'[sah]T
(mo! dm"")
0.0
1.0
2.5
5.0
7.5
10.0
12.5
15.0
20.0
[Ce(IV)]T
[L(-)sorbose]
[H2S04]T
[CTABJT
Temperature
IT
lO'kvp
NajSO,
4.7
4.2
4.0
3.8
3.4
3.2
3.0
2.8
2.6
= 1.0x
= 4.0x
10"
10-
•5
mol dm
mol dm~
•- 1.83moldm"^
= 50.0}
= 40°C
(s-)
4
<10' 1
" mol dm
NaNOj
4.7
4.4
4.2
4.2
4.1
4.0
3.8
3.8
3.5
NaCl
4.7
4.5
4.3
4.3
4.1
4.0
4.0
3.8
3.8
120
w
o
10.0
lO^C salt D(mol d m - ^ )
Fig. 3.19: Effect of [saltjx (i^&iSO^ ( • ) , NaNOj (V) and NaCl (A)) on the
rate of oxidation of D(+)xylose by cerium(I V) in presence of CTAB
(= 50.0 X 10" mol dm~ ). Conditions: [Ce(IV)]T = 1.0 x 10" mol
dm-\ [D(+)xylose]T = 4.0 x 10~ mol dm"^ [H2S04]T = 1.83 mol
dm~^ temperature = 40 °C.
121
10.0
lO^CsaltDlmoldm-^ )
20.0
Fig. 3.20; Effect of [salt]T (NaiSO^ ( • ) , NaNOj (V) and NaCl (A)) on the
rate of oxidation of L(+)arabinose by cerium(IV) in presence of
CTAB (= 50.0 X 10" mol dm"^). Conditions: [Ce(IV)]T - 1.0 x 10"
mol dm"^ [L(+)arabinose]T = 4.0 x 10" mol dm~\ [H2S04]T = 1.83 -3 mol dm , temperature = 40 °C.
122
10.0
lO^CsQlQImoldm-S )
2 0.0
Fig. 3.21: Effect of [saltlx (Nsn^O^ ( • ) , NaNOa (V) and NaCl (A)) on the
rate of oxidation of D(+)glucose by cerium(IV) in presence of
CTAB (= 50.0 X 10" mol dm~ ). Conditions: [Ce(IV)]T = 1.0 x 10"
mol dm"^ [D(+)glucose]T = 4.0 x 10~ mol dm"\ [H2S04]T - 1.83
mol dm~ , temperature = 40 °C.
123
0.0 10,0
10^ C s o l t ; ] (mol (Jm"3 )
20.0
Fig. 3.22: Effect of [saltji (Na2S04 ( • ) , NaNOj (V) and NaCl (A)) on the
rate of oxidation of D(+)mannose by cerium(IV) in presence of
CTAB (= 50.0 X 10~ mol dm"0- Conditions: [Ce(IV)]T = 1.0 x 10 -3
-2 -3 mol dm'", [D(+)mannose]T = 4.0 x 10"' mol dm"', [H2S04]T = 1.83 -3 mol dm , temperature = 40 °C.
124
Z L.O-
10.0
10^ C so l tD (moldm" '^ )
Fig. 3.23: Effect of [saltji (Na2S04 (• ) , NaNOj (V) and NaCl (A)) on the
rate of oxidation of D(-)fructose by cerium(IV) in presence of
CTAB (= 50.0 X 10' mol dm"^). Conditions: [Ce(IV)]T = 1.0 x 10"
mol dnf\ [D(-)fructose]T = 4.0 x 10~ mol (inf\ [H2S04]T = 1.83
mol dm^^ temperature = 40 °C.
125
in
o
4.0
30
?.o
^ ^
b::;^;^ .. 4
^**™i
... 1 1 0.0 10.0
10 Csalt 3 Imoldm"^)
20.0
Fig. 3.24: Effect of [saltjj (Na2S04 ( • ) , NaNOa (V) and NaCl (A)) on the
rate of oxidation of L(-)sorbose by cerium(IV) in presence of CTAB
(= 50.0 X 10" mol dm~ ). Conditions: [Ce(IV)]T = 1.0 x 10" mol
dm"\ [L(-)sorbose]T = 4.0 x 10" mol dm~^ [H2S04]T = 1.83 mol
dm~^ temperature = 40 °C.
126
B. Discussion
The mechanism
(a) in the absence of CTAB
Evidence for the formation of stable co-ordination complexes has been
obtained by kinetic and spectrophotometric methods for cerium(IV) oxidations
of many compounds in perchloric and nitric acids.*'""'^ Complex formation in
cerium(IV) oxidations in sulfuric acid solutions, however, has been indicated
only in a few studies,''*''^ and in none of these was spectrophotometric evidence
provided for the complex. Thus, in aqueous H2SO4 media, complexation of
organic substrate with Ce(IV)—sulfato species is a rare possibility.'^''^ However,
there may be a weak association and the following steps given in Scheme 3.1 are
quite reasonable.
K Ce(IV) + S • complex (C) (3.3)
C • Ce(III) + S*+ H^ (3.4)
S* + Ce(IV) ^ ^ > products + Ce(III) + H^ (3.5)
Scheme 3.1
The above Scheme is similar to that proposed by Mehrotra et al Here,
the complex C unimolecularly disproportionates in the rate determining step to
give Ce(III) and free radical S".
127
A second mechanism assumes that the substrate is directly oxidized by
Ce(IV) in a bimolecular step. In this case, the interaction may occur at the
transition state (Scheme 3.2).
Ce(IV) + S • {Ce(IV) S} • Ce(III) + S* + H^ (3.6)
Scheme 3.2
When the K-value is very small then it is not possible to discriminate
between Schemes 3.1 and 3.2. In the above Schemes, Ce(IV) actually denotes
the kinetically active Ce(IV)-species.
Active species of reductant
Monosaccharides, especially those with five and six carbon atoms,
normally exist as cyclic molecules rather than as the open-chain forms. The
cyclization takes place as a result of interaction between the functional groups on
distant carbons to form a cyclic hemiacetal (in aldoses) or a cyclic hemiketal (in
ketoses). In either case, the carbonyl carbon becomes a new chiral center called
the anomeric carbon. The cyclic sugar can take either of two different forms,
designated a and p, called anomers of each other. A five-membered ring is called
a fiiranose because of its resemblance to furan; a six-membered ring is called
pyranose because of its resemblance to pyran. The pyranoid form is generally
the most stable and exists mainly as a chair form. Out of these, only the pyranoid
form is claimed to be involved in the oxidation reactions.'^ The |3-form having
128
—OH group at C-1 at the equatorial position is more reactive than the a-form
(—OH axial). °
R R
vi^^^'^ ^^ w—^0" OH X
(a-anomer) (p-anomer)
R = —H (for aldopentoses and ketohexoses) or
—CH2OH (for aldohexoses)
X = — H (for aldopentoses and aldohexoses) or
—CH2OH (for ketohexoses)
Active species of oxidant
Cerium(IV) oxidations in perchloric acid are generally fast whereas the
sulfuric acid medium has been widely used to investigate the reaction with
organic substrates, where the eerie species acts as an one-electron oxidant. The
primary products of such reactions are radicals which subsequently undergo
rapid oxidation to stable species. It is well known that different types of
complexes CeS04^^ Ce(S04)2, Ce(S04)3^", H3Ce(S04)4", HCe(S04)3", and
Ce(OH) " are present in a eerie sulfate solution and that their relative
concentration is a function of the pH and sulfate ion concentration. ' '* '*
129
On the basis of rate inhibitory effects of H2SO4 and HS04~, Ce(S04)2 has
been considered to be the reactive form.' '' '' ^ On the other hand, HS04~ and
H2SO4 dependencies also suggest the participation of HCe(S04)3~ and
H3Ce(S04)4", respectively,' ' ' as the reactive cerium(IV) species. In our study,
the reaction is catalyzed by CTAB while anionic SDS has no effect. This clearly
indicates the involvement of a negatively charged species of cerium(IV) and
excludes the possibility of Ce(S04)2 being the reactive species (attraction and
repulsion, respectively, between the negatively charged cerium(IV) species and
the head groups of the two types of surfactant micelles seemingly play important
role in the present case). Thus, it may be considered that Ce(S04)3 , HCe(S04)3 ,
and H3Ce(S04)4~ could be the reactive species in the present systems, but, the
dependency on [H2SO4] (inverse-order) indicates that H3Ce(S04)4" can not be
the active form. Previous observation of Hardwick and Robertson^ suggested
that ca. 93% of the cerium(IV) in IN H2SO4 is present as Ce(S04)3^" and
proposed the equilibrium Ce(S04)2 + HS04~ ^^=^ €6(804)3^' + H^ but
the formation of Ce(S04)3^" has been questioned. '' ^ For argument sake, if we
do consider Ce(S04)3^~ as the reactive species, simultaneous formation of
HCe(S04)3" is equivalent to the formation of Ce(S04)3^~ + H" in solution.
Presently, it is difficuh to answer as which of the two negatively charged species
(Ce(S04)3^" or HCe(S04)3") is the active form of cerium(IV) (also, at [H2SO4] =
1.83 mol dm"^ the fractions of these species are high: 2.46 x 10" (Ce"" ), 4.71 x
10^ (CeS04^^), 0.051 (Ce(S04)2), 0.562 (Ce(S04)3^-), 0.184 (HCe(S04)3"),
0.201 (H3Ce(S04)4~)).
130
On the basis of observed results (first-order with respect to oxidant and
reductant) and these arguments, a general mechanistic scheme may be proposed
depicting the behavior of carbohydrates towards the oxidant cerium(IV).
Schemes 3.3 and 3.4 explain, respectively, the oxidation of aldoses and
ketoses.
R O
.OH + Ce(IV) K, cl p-anomer Ce(IV)
H
(3.7)
(3-anomer
Cl —L_^
Cl R
Radical + Ce(IV)
O
6 + Ce(III) + H^
H Radical
fast
R
Y^,,,^.^' '*^ + Ce(III) + H*
O Lactone
(3.8)
(3.9)
OH/H2O
H"
alkaline NH2OH
FeCb +HC1
Blue cobur
R OH
^O
Aldonic acid anion
FeCb + phenol
Yellow cobur
(3.10)
Scheme 3.3
131
v \
n V ^ 1
H
, _ . ^ - ^ ^ ^ \ PH + Ce(IV) _ M _
CH2OH -
(3-anomer
H
Lactone ^
p-anomer Ce(IV)
CI
CH2OH + H^
Radical
(3.11)
(3.12)
Radical + Ce(lV) _J!5L^ Ce(Ill) + HCHO + H* ^^'^^
Scheme 3.4
The first step in Scheme 3.3 is the complex formation between the
reactive species of cerium(IV) and P-anomer. The redox decomposition of this
complex C1 in the subsequent rate-determining step gives rise to free radical and
Ce(III) (Eq. (3.8)). The lactones are the oxidation products of aldoses in
accordance with Eq. (3.9).
In the oxidation of ketoses (Scheme 3.4), the first step is same as that in
case of aldoses, i.e., the formation of a complex. The second step is the rate
determining step leading to the formation of lactone, free radical and Ce(III).
The free radical combines with Ce(IV) to produce Ce(III) and formaldehyde.
132
The rate determining steps in both the Schemes 3.3 and 3.4 are similar.
Therefore, the rate law for both aldoses and ketoses should also be similar which
can be given as
-d[Ce(IV)] = k,Kc,[carbohydrate] [Ce(IV)] (3.14)
dt
The rate law (Eq. (3.14)) is in complete accord with the observations, i.e.,
first-order dependence both in [carbohydrate] and [Ce(IV)] (as explained above,
exact equation for observing the inverse-order kinetics each in [H2SO4] and
[HS04~] cannot be derived due to uncertainty of the involved protonic equilibria
producing the active Ce(IV) species).
The oxidation kinetics of carbohydrates by cerium(IV) proceed in two
stages, i.e., initial slow stage followed by a relatively faster step. The mechanism
of first step is discussed here. The second step oxidation (autocatalytic reaction
path) is not a true reaction path for the oxidation of carbohydrates by
cerium(IV): it may be a mixture of the rates of carbohydrates and their oxidation
products (lactones and aldonic acids). Sala and coworkers studied the oxidation
of lactones by chromium(VI) and reported that the rates of their oxidation are at
least 10-fold higher in comparison to corresponding monosaccharides. ' ^
Thus, it can be concluded that under the present kinetic conditions, the exact
stoichiometry and product analysis cannot be predicted and the exact
dependence of the autocatalysis path on reactant concentrations cannot be
estimated.
133
(b) in the presence of CTAB
Micellar aggregates composed of single chain surfactant molecules are
the simplest of the dynamic aggregates to affect catalysis. There are numerous
reactions in which solubilization of one or more of the reactants in the micellar
aggregates leads to significant alteration in the reaction rates. Solubilization
introduces two new situations that can influence the reaction rates: alteration in
the local distribution of the solute (reactants) and surface/interface effects.
It has already been mentioned that the reactions of cerium(IV) and
carbohydrates are catalyzed by cationic micelles of CTAB while anionic
micelles of SDS have no effect on the rates. The dependencies on variables like
[reductant], [oxidant], [H2SO4], and temperature are same both in aqueous and
CTAB media. Therefore, it can be concluded that the mechanisms of redox
reactions of cerium(IV) and carbohydrates remain the same in both the media.
Lower values of activation energy in presence of CTAB as compared to
reactions in aqueous medium (Tables 3.25-3.30) clearly indicate that the CTAB
acts as catalyst and provides a new reaction path with lower activation energies.
However, a complete account of all the factors that influence AH** and AS is not
possible because the rate constant k^ does not represent a single elementary step
(as it being a complex function of k'w, k'm, and Ks, Scheme 3.5). Furthermore,
though a change in temperature is known to produce changes in size, shape,
surface charge, etc. of the micelles, equally good fit of the observed data (kobs
134
and k4/) to the Eyring equation both in the absence as well as presence of
surfactants shows that the micelle structural sensitivity to temperature is
kinetically unimportant. Similar conclusions had been drawn earlier also. ' '
The kinetic results of the effect of varying [CTAB] on the rates of
reactions can be explained by means of pseudophase model proposed by Menger
and Portnoy, which takes into consideration solubilization of one reactant only
into the micellar phase. The variation of rate constants with surfactant
concentration is treated on the assumption that the substrate (the kinetically
active Ce(IV)-species) is distributed between the aqueous and micellar
pseudophases as given in Scheme 3.5 ((Ce(IV))m is the micellized Ce(IV)-
species, [Dp] = [surfactant]! - cmc, Ks is the micelle—Ce(IV) binding constant
and k\v and k'm are the respective first-order rate constants in aqueous and
micellar pseudophases).
Ks (Ce(IV)), + Dn = ; = ^ (Ce(IV))^
k'\v\carbohydrate k' /carbohydrate
Products
Scheme 3.5
According to Scheme 3.5, the observed rate constant is expressed as a function
of[D„],Eq.(3.15), 2
135
k'^+k',KJD„] k4- = (3.15)
1 + Ks [Dn]
On rearranging, Eq. (3.15) gives Eq. (3.16)
1 1 + (3.16)
(k'w - k4.) (k\ - k'J (k- - k 'J Ks [Dn]
n-l Equation (3.16) predicts that a plot of left-hand side versus [Dn]~ should be
linear. When the data were fitted into Eq. (3.16), good linear plots were observed
(Figs. 3.25-3.30) implying that the Scheme 3.5 model is suitable to explain the
observed catalytic role of CTAB micelles. The values of Ks and k'm were
calculated for each carbohydrate from the slopes and intercepts of the respective
plots of l/(k\v- k /) versus l/[Dn]. These values are given in Table 3.43. Kj, k'm
and [Dn] were used to recalculate rates (kvpcai, Tables 3.31-3.36) which are in
good agreement with the observed k^, this confirms the validity of rate law Eq.
(3.15) and the Scheme 3.5 model.
Probable reaction site
All micellar-mediated reactions are concluded to occur in the Stem
layer. ^ Micellar surfaces are water rich '* (activity of the water at the surfaces of
ionic micelles is not different from water activity in the aqueous pseudophase).
Electrostatic and hydrophobic interactions play important role in the
136
1 0 " ^ / C DnD (mo r ' ' dm3 )
Fig. 3.25: Plot of \/{k'^- k^) versus l/[Dn] for the oxidation of D(+)xylose by
cerium(IV) in presence of CTAB (= 50.0 x 10" mol dm"^).
Conditions: [Ce(IV)]T = 1.0 x 10" mol dm'^ [D(+)xylose]T = 4.0 x
10~^ mol dm"^ [H2S04]T = 1.83 mol dm"^ temperature - 40 °C.
137
V)
I
10-VCDnD(mol dm3)
Fig. 3.26: Plot of l/(k'w- ks ) versus l/[Dn] for the oxidation of L(+)arabinose
by cerium(IV) in presence of CTAB (= 50.0 x 10" mol dm" ).
Conditions: [Ce(IV)]T = 1.0 x 10~ mol dm"^ [L(+)arabinose]T = 4.0
X 10~ mol dwr\ [H2S04]T = 1.83 mol dm"^ temperature = 40 °C.
138
10.0 -
0.0 5.0 10.0
10"VCDnD(mol- ' 'dm3)
Fig. 3.27: Plot of l/(k'w- k ) versus l/[Dn] for the oxidation of D(+)glucose by
cerium(IV) in presence of CTAB (= 50.0 x 10" mol dm~ ).
Conditions: [Ce(IV)]T = 1.0 x 10" mol dm"\ [D(+)glucose]T = 4.0 x
10" mol drvr\ [H2S04JT = 1.83 mol dm"^ temperature = 40 °C.
139
5.0 lO'VCOnlKmoHdm^)
Fig. 3.28: Plot of l/(k'w- k^) versus \/[D„] for the oxidation of D(+)mannose
by cerium(IV) in presence of CTAB (= 50.0 x 10~ mol dm" ).
Conditions: [Ce(lV)]T = 1.0 x lO"' mol dm"\ [D(+)mannose], = 4.0
X 10" mol dm"\ [H2S04]T = 1.83 mol dm"\ temperature = 40 °C.
140
1 0 " V C D n l (mor''dm3)
Fig. 3.29: Plot of I/(k'w- k*}/) versus \/[D„] for the oxidation of D(-)fructose by
cerium(IV) in presence of CTAB (= 50.0 x 10""' mol dm~ ).
Conditions: [Ce(IV)]T = 1.0 x 10~ mo! dm~\ [D(-)fructose]T = 4.0 x
10~ mol dm~^ [H2S04]T = 1-83 mol dm"^ temperature = 40 °C.
141
to
I
1
O T
10.0
10~^/COn3(mor l dm3)
Fig. 3.30: Plot of l/(k'w- k4/) versus l/[Dn] for the oxidation of L(-)sorbose by
cerium(IV) in presence of CTAB (= 50.0 x 10' mol dm"^).
Conditions: [Ce(IV)]T = 1.0 x 10' mol dm"' [L(-)sorbose]T = 4.0 x -3 -3 10~' mol dm"", [H2S04]T = 1-83 mol dm~', temperature = 40 °C
142
TABLE 3.43:
Values of micelle-cerium(IV) binding constants (Ks) and rate constants in
micellar medium (k'm) for the oxidation of carbohydrates by cerium(IV) in
presence of CTAB.
Conditions: [Ce(IV)]T = 1.0 x 10' mol dm"
[carbohydrate]! = 4.0 x 10' mol dm ^
[H2S04]T =1.83 mol dm~
Temperature = 40 °C
Carbohydrate
D(+)xylose
L(+)arabinose
D(+)glucose
D(+)mannose
D(-)fructose
L(-)sorbose
Ks
(mol"'
89.0
165.3
52.1
58.2
191.0
99.9
dm )
10'kV
(s-^)
1.7
2.2
1.1
1.5
3.8
3.2
10'k'^
(s-')
4.2
5.0
3.1
4.8
6.6
7.6
'k'w= kobs, obtained under similar conditions without added CTAB (see Tables 3.1-3.6)
143
incorporation/association of substrates into micelles. Many ionic species of
cerium(IV) are believed to be present in the aqueous H2SO4 medium. ' ' The
positive catalytic effect of CTAB micelles indicates that the chemically active
species must be anionic which may get associated through electrostatic
interaction with the positive head group of CTAB micelles. The second reactant,
carbohydrate, has no hydrophobicity due to the presence of 4 or 5 hydrophilic
—OH groups. As the reaction proceeds through the formation of a complex (see
Eq. (3.7)/(3.11)), the associated Ce(IV)-species may form complex Ci at the
Stem and Gouy-Chapman layers' junctural region " ^ (see Fig. 3.31 of a
possible arrangement). The complex, having negative charge, may now orient in
a manner suitable for proceeding the reaction further.
Effect of SDS micelles
It is evident from the data depicted in Tables 3.31-3.36 and Figs. 3.13-
3.18 that SDS has no effect on the reaction rate. The negative head groups of
SDS micelle simply repel the anionic species of cerium(IV). On the basis of
these observations our conclusion that, out of various sulfate species of
cerium(IV), only those with negative charge are the reactive species seems
logical.
Effect of [salt] on the k^>
When salts are added to micellar solutions, the counterions of the salts
compete for the ionic head group of micelles with the surfactant counterions
144
Br
H3C-N>-CH3
Fig. 3.31: Schematic model showing probable location of reactants for the ionic
micellar catalyzed redox reaction between cerium(lV) and
carbohydrates.
145
that already exist in solution. Thus, displacement can occur, depending on the
relative affinities of counterions for the head groups. Kinetic salt effects are
peculiar in micellar systems'*'' and interesting kinetic effects can arise. If the
added salt is reactive, micellar rate enhancements are observed after
displacement. Inert salts, especially the inorganic ones, and organic additives
have generally been found to decrease the rates of reactions.
The salt effects are specific and depend upon the nature of the ion which
has a charge opposite to that of the micelle, suggesting that both electrostatic and
hydrophobic factors play a role. The effects are greatest for large, low charge
density and hydrophobic ions which interact most strongly with the reactive
counterionic micelles. In general, the more hydrophobic a character possessed
by the ion, the better inhibitor it becomes. The inhibitory effect of an ion
increases with its ability to lower the critical micelle concentration and surface
potential, to increase aggregation number, and to decrease the ionization degree
of micelles. " ^
It has been reported that upon the addition of salts or organic solvents,
micellar surfaces become more hydrated and the water content of the micellar
interface plays an important role in inhibition.'**''*
Our results of the effect of inorganic electrolytes (NaCl, NaNOs and
Na2S04) on the CTAB-catalyzed oxidation of carbohydrates by cerium(IV) show
that the added salts inhibit the rates of reactions (Tables 3.37-3.42 and Figs.
3.19-3.24). Each anion is an inhibitor and the inhibitory power increases in the
146
order: CI" < NO3" < S04^'. As the concentration of these electrolytes increases,
the concentration of Ce(IV) at the reaction site decreases (the counterions of the
added salts compete with the reactive Ce(IV)-species for the CTAB head group
region). Our kinetic salt effects thus suggest that some additional factors than
changes in micellar size and shape are involved in the present system. The
inhibitory effect may thus, at least in part, be due to the exclusion of reactive
species of cerium(IV) from the reaction site.
Comparison of second-order rate constants
Table 3.44 summarizes the values of second-order rate constants (k", mof' dm^
s~') for the reactivity of carbohydrates, used in this study, with cerium(IV).
These results indicate that presence of —OH, —CHO and ketonic groups
increase the reducing power in the order aldohexoses < aldopentoses <
ketohexoses. The trend shows that the oxidation by cerium(IV) seemingly
depends on the number of —OH groups, stereochemistry and the chelating
ability of the monosaccharides. D(-)fructose has greater tendency to reduce
cerium(IV) in comparison to L-sorbose and other monosaccharides (L(-)sorbose
> L(+)arabinose > D(+)xylose > D(+)mannose > D(+)glucose). It is interesting
to note that the oxidation rate of various monosaccharides studied are of the
same order. This means that these sugars are oxidized by a common mechanism,
i.e., cerium forming a complex with C-1 hydroxyl group of the sugar prior to its
rate-limiting disproportionation to a free radical.
147
TABLE 3.44
Second-order rate constants for the oxidation of carbohydrates by cerium(IV) in
absence (k") and presence (kvp") of CTAB.
Conditions: [Ce(IV)]T = 1.0 x 10" mol dm"
[carbohydrate]! = 4.0 x 10" mol dm"''
[H2S04]T =1.83 mol dm"
Temperature = 40 °C
Carbohydrate 10'k'Vk%(mor'dmV')
Aqueous CTAB'
D(+)xylose 4.2 6.3
L(+)arabinose 5.5 9.5
D(+)glucose 2.8 4.3
D(+)mannose 3.8 6.0
D(-)fructose 9.5 13.0
L(-)sorbose 8.0 11.8
' [CTAB]T = 50.0 X 10"* mol dm
148
References
1. D. Grant, D. S. Payne, Anal. Chim. Acta, 1961, 25, 337.
2. D. Grant, J. Inorg. Nucl. Chem., 1964, 26, 337.
3. L. J. Heidt, M. E. Smith, J. Am. Chem. Soc, 1948, 70, 2476.
4. B. Pare, M. Pipada, A. Choube, V. W. Bhagwat, Oxid. Commm., 2003,
26, 95.
5. Z. Khan, Raju, M. Akram, Kabir-ud-Din, Int. J. Chem. Kinet., 2004, 36,
359.
6. T. J. Hardwick, E. Robertson, Can. J. Chem., 1951, 29, 828.
7. F. R. Duke, F. R. Parchen, J. Am. Chem. Soc, 1956, 78, 1540.
8. H. L. Hintz, D. C. Johnson, J. Org. Chem., 1967, 32, 556.
9. R. Dayal, G. V. Bakore, Indian J. Chem., 1972,10, 1165.
10. S. B. Hanna, S. A. Sarac, J. Org Chem., 1977, 42, 2063.
11. F. R. Duke, R. F. Bremer, J. Am. Chem. Soc, 1951, 73, 5179.
12. S. S. Muhammad, K. V. Rao, Bull. Chem. Soc Jpn., 1963, 36, 943..
13. C. R. Pottenger, D. C. Johnson, J. Polym. ScL: PartA-I, 1970, 8, 301.
14. J. S. Littler, J. Chem. Soc, 1959, 4135.
15. R. N. Mehrotra, Z phys. Chem., 1965, 230, 221.
16. W. H. Richardson, In ''Oxidation in Organic Chemistry, Part A, K. B.
Wiberg (Ed.), Academic Press, New York, 1965.
17. A. K. Das, Coord Chem. Rev., 2001, 213, 307.
18. R. N. Mehrotra, E. S. Emis, J. Org Chem., 1974, 39, 1788.
149
19. M. Rudrum, D. F. Shaw, J. Chem. Soc, 1964, 52.
20. A. S. Perlin, Can. J. Chem., 1964, 42, 2365.
21. L. T. Bugaenko, H. Kuan-Lin, Russ. J. Inorg. Chem., 1963, 8, 1299.
22. S. E. Kharzeeva, U. V. Serebrennikov, Russ. J. Inorg. Chem., 1967, 12,
1601.
23. P. S. Sankhala, R. N. Mehrotra, J. Inorg Nucl. Chem., 1972,34, 3781.
24. S. A. Chimatadar, S. T. Nandibewoor, M. I. Sambrani, J. R. Raju, J.
Chem. Soc, Dalton Trans., 1987, 573.
25. A. K. Das, S. K. Mondal, D. Kar, M. Das, Inorg React. Meek, 1999, 1,
169.
26. A. Aganval, G. Sharma, C. L. Khandelwal, P. D. Sharma, Inorg. React.
Mech., 2002, 4,223.
27. S. A. Chimatadar, T. Basawaraj, S. T. Nandibewoor, Inorg. React.
Mech., 2002, 4, 209.
28. S. I. Garcia, S. R. Signorella, S. Acebal, E. Piaggio, L. F. Sala. Oxidn.
Commun. 1993,16,313.
29. S. R. Signorella, M. Santoro, C. Palopoli, J. M. Brondino, S. Peregrin,
M. Quiroz, L. F. Sala, Polyhedron, 1996,17, 2739.
30. G. Cerichelli, L. Luchetti, G. Mancini, G. Savelli, C. A. Bunton, J.
Colloid Interface Sci., 1993,160, 85.
31. Kabir-ud-Din, J. K. J. Salem, S. Kumar, Z. Khan, Colloids Surf. A:
Physicochem. Eng. Aspects, 2000,168, 241.
32. F. M. Menger, C. E. Portnoy, J. Am. Chem. Soc, 1967, 89, 4698.
150
33. E. H. Cordes, R. B. Dunlap, Ace. Chem. Res., 1969, 2, 329.
34. F. M. Menger, Ace. Chem. Res., 1979,12, 11.
35. V. K. Grover, Y. K. Gupta, J. Inorg. Nucl. Chem., 1969, 31, 1403.
36. S. K. Mishra, Y. K. Gupta, J. Chem. Soc. A, 1970, 2918.
37. A. K. Das, S. K. Mondal, D. Kar, Indian J. Chem., 1998, 37A, 1102.
38. Kabir-ud-Din, A. M. A. Morshed, Z. Khan, Carbohydr. Res., 2002, 337,
1573.
39. Kabir-ud-Din, A. M. A. Morshed, Z. Khan, Inorg. React. Mech., 2002,
3, 225.
40. Kabir-ud-Din, A. M. A. Morshed, Z. Khan, Int. J. Chem. Kinet., 2003,
35, 543.
41. Kabir-ud-Din, A. M. A. Morshed, Z. Khan, Oxid. Commun., 2003, 26,
59.
42. Kabir-ud-Din, A. M. A. Morshed, Z. Khan, J. Carbohydr. Chem., 2003,
22, 835.
43. Kabir-ud-Din, A. M. A. Morshed, Z. Khan, Indian J. Chem., 2004, 43B,
2178.
44. S. Tascioglu, Tetrahedron, 1996, 52, 11113.
45. M. S. Famandes, P. Fromherz, J. Phys. Chem., 1977, 81, 1755.
46. I. Ascone, P. D'Anglo, N. V. Pavel, J. Phys. Chem., 1994, 98, 2982.
47. M. R. R. Almgren, J. Phys. Chem., 1979, 83, 360.
48. D. Grand, J. Phys. Chem., 1990, 94, 7585.
49. L. Kevan, Proc. DOE, Sol. Photochem. Res. Conf., 14*^ 1990, 69.